Vehicle Purpose & Cost Effectiveness
Diesel swaps are not generally performed for cost effectiveness (but this will depend on what kind of swap you are engaged in). You may likely end up with a vehicle that obtains significantly better fuel economy, but how long will these savings take to pay off your initial investment? Make sure you are performing the swap for the right reasons. If the money spent and time invested is worth the end result, then get to work.
Additionally, ask yourself how you plan on using the vehicle. Just because you re-power a 1/2 ton with a heavy duty diesel does not mean you can tow more - you must obey the original equipment manufacturer's ratings. You may, however, tow with less effort, and with less fuel burned. Swapping a Cummins into a Super Duty provides a huge "wow factor", but there's no certainty this modification will pay for itself in the long run. Re-powering a 1980's Chevy K10 with a 6.2L diesel, on the other hand, is much more straight forward, since the engine was originally optional for the vehicle and may yield significant fuel savings. At the same time, don't expect a 6.2L to pull like a big block gasser.
The first tool you pick up should be a tape measure, not a wrench. Make sure the engine is going to physically fit in your vehicle. If your engine bay is too small for the engine you have selected, examine what it would take to get it to fit. Can the firewall be extended? Will adding a hood scoop help clear the engine? Don't forget to take into consideration the length of the fan and accessories. Electric cooling fans are also an option to save space. You will likely need to install (and therefore clear) a larger radiator, since most radiators designed for gasoline applications are smaller than what a comparable oil burner will require.
Your diesel is likely going to be vastly heavier than any gasoline engine it replaces. You will need to ensure the frame, front suspension, and front axle can handle this additional load. Boxing the front section of the frame is an option for increasing structural integrity. Suspension can also be upgraded to support additional weight (stiffer coils/leaf springs). In some cases, swapping the front suspension and axles from a 3/4 or 1 ton vehicle may be the easiest option, but this depends on what kind of vehicle you're dealing with. Don't forget that you will most likely need motor mounts designed for the engine you are installing (there are exceptions, as in the 6.2L/6.5L GM, which uses the same engine mounts as Chevrolet/GMC V8 gas engines).
Transmission & Rear Axle
If the diesel being used in your conversion is an older 6.2L/6.5L GM, 6.9L/7.3L IDI, or 12v Cummins that you plan on keeping stock, you may be able to get away with 1/2 axles. But let's be honest, 200 hp just doesn't cut it for most of us. If you plan on modifying the engine, whether it be 100 hp over stock or 500 hp over stock, you'll need to address the drivetrain. A 1/2 ton axle (or even lighter) will not survive the torque output of a potent diesel, so source a 3/4 or 1 ton axle. Some 1/2 ton axles, the Ford 9 inch for example, can be built to handle extreme power levels. However, the most economical route would be source a stronger full floating axle.
A modified gasoline transmission will not suffice behind a diesel whose power levels are well above those of the engine it was designed to transmit power for. At the very least, source the transmission that the engine original came with. The amount of work the transmission will need depends on your desired hp/torque output, but plan on rebuilding and upgrading the transmission if you're performing a diesel swap for performance reasons. Obviously, swapping a manual transmission is a much easier and less expensive task. Make sure the stick shift was designed for a diesel, and put a performance clutch in front of it. Don't forget to make sure the transmission will fit. Transmission tunnels can be fabricated to allow larger transmissions to be installed. The final drivetrain consideration is gearing, for maximum performance and fuel economy you may want to switch axle ratios accordingly.
if you've chosen to swap a mechanical engine (12v Cummins or any of the old IDIs), you can skip this section, as you've made it easy on yourself. If you're dropping a 24v Cummins common rail and Allison transmission into a 65 F-100, you'll need to address the electronics. Electronically controlled diesel engines and transmissions communicate with one another. A Ford transmission can be modified to speak to a Cummins engine, an Allison transmission can be programmed to speak to a Power Stroke engine, etc.
Obviously, selecting an engine and transmission from the same manufacturer simplifies the task. There are 2 basic options when it comes to vehicle electronics. 1) Pull all the electronics, including gauges, PCM, etc, from your donor vehicle and swap them into your conversion vehicle. 2) Purchase a stand-alone electronics system for your engine. Both options will yield the same end results. Pulling the electronics from a donor vehicle will prove easier and much less expensive, though the popularity of using stand-alone electronics is on the rise.
Don't plan on swapping a diesel into any vehicle that requires a SMOG or similar inspection (applies to California and other states that have adopted California's strict emissions laws). For CA residents, that means all gas vehicles built after 1975. We're not even sure what the SMOG technician would say when he popped the hood, but it won't be good. While there may be exceptions and loop holes, it could be a nightmare of paperwork. If you're going to put your time and money into a diesel swap, be sure to check your local laws prior to investing in your project.
If you are performing a diesel swap, consider obtaining an entire donor vehicle that has a usable engine, drivetrain, and electronics. You will be money ahead if you source the components you need from a donor (even a totaled, junked, or salvaged vehicle as long as it has some usable components) and then part it out to help cover your costs. If this is not an option, don't be discouraged from doing a diesel conversion. We simply recommend you plan out the entire build addressing all the obstacles you may face before you begin. Remember, the obstacles/hardships you will face depend on your project. Swapping a 4BT Cummins into a 1/2 ton Chevy is a completely different undertaking than dropping a common rail Cummins into a Jeep.