1. Depending on how the frame is made it is sometimes possible to re-drill the bolt holes for the rear suspension farther forward, move the suspension forward and then cut off the extra tail of the frame. No welding.
2. If you would like a diesel it will be cheaper and easier to buy a truck with one in it.
If you want a Diesel in the 63 see if the factory ever put one in, that would be the easiest swap.
If you find an engine you like and want to swap it it can be done with out much trouble, assuming it actually fits, but it will take you some time and lots of measuring and figuring.
Depending on what you want to haul many different engines may suit your needs. While you figure that out I would recommend staying away from a Cat 3208, a Detroit 8.2, and a GM 6.2, and possibly the 6.5 too. You may want to consider an international DT466, a Cummings 6 cylinder, sized to your load, a Detroit 4-53t or 6v53t, or 4-71T.
3. What you need to change depends on the engine you pick, how fast you want to go, where you drive, what transmission, and what rear ratios you have.
Once you know your tire size, have sized the engine HP to the load and speed, and know at what RPM that engine is most efficient you can use a calculator like this to choose the OD ratio or DD you need in your transmission to work with the rear axle you have or will swap.
http://www.drivetraindirect.com/t_gear_calculator.htm If your current transmission has a round clutch housing with 12 bolts it is compatible with a diesel engine.
4. It will be a pain and will cost more money than you ever thought. Depends on what you want, and how bad you want it. You have asked some good questions, so you have been thinking about this, but i get the impression you are not sure you really want to do this yet. Don't start unless you are.
Grigg
