[dsraven]

questions:
-2wd or 4x4?
-intended amount of cab stretch
-track width of old vs new donor truck
-cab mounting widths of old cab vs frame width of new donor frame
-are your fab skills, shop space and toolbox able to do the work
-do you want the wheels to be sticking out or tucked in like normal
-have you thought of using the donor truck floor and firewall as a unit. that enables seat mounts for newer style seats with shoulder belts integrated, trans hump, brake booster mount, heater etc etc

using the stock frame with extension on a long truck and adding more power than it had stock may be asking for trouble. it will likely have some thin spots due to rust under cross member locations or old shock mounts etc. the old frame is likely tapered skinny at the front and wide at the rear which can make it a challenge to fit parts on as the frame gets longer and the rear part would get wider unless tapered back in, it may be better for you to find a newer truck with all the gear you need and use that as a donor. buy a complete running truck that has the power plant you want. that will give you a powertrain that fits the whole system, more modern suspension with easily found replacement parts, larger power brakes all around most likely without any upgrades and a master cylinder made for the brake system, all the fuel and brake lines will be in place, a fuel tank with integrated fuel pump etc as applicable, exhaust that already fits past the frame rails and steering parts (except for the steering column mod required to fit your cab configuration) and is upgradable easily by choosing an off the shelf part if you want to. keep in mind the wheel size diameter and gear ratio the donor truck had as this affects the engine rpm and speedo reading if you change that diameter. long story short, do a pro and con list for both directions of build as well as a list of things you would need to do in both directions of build. estimate some time and money and then go from there. just remember to build it safe, not just safe for you but safe for all the other people on the road. I have heard some guys say "it's me driving so if something goes wrong it is just me that gets hurt or killed". that may be true but only if you drive on the moon where there are no other drivers or families on the road.
anyway, get a plan going and try to stick to it. the longer the project goes the more chance you will change your mind on something and the project will end up going in a different direction or be sidelined because you get over your head or can't figure out what to do next. each time costing money, time and leaving a pile of parts leftover or a for sale sign hung. some guys will go as far as making a sketch of the frame and how the suspension will work, how the body will mount, the dimension between the engine and the rad, how much the donor frame will need to be cut, how the wheels will look in fender openings etc. like a pro shop would do. not a bad idea actually. flying by the seat of the pants usually means spending more time figuring things out and changing your mind a few times.