Ssedan is trying to steer you in the right direction. You are going to spend a lot of money getting that 700R4 into the vehicle. Factor in new transfer case, potentially cutting both driveshafts to get correct length, TV cable, tv cable bracket, Geometry corrector or new linkage to attach TV cable to induction system, moving the crossmember, reworking the mount location, bending or new cooler lines. Then, when it is all said and done, you will have a transmission that will be HALF as strong as the trans you have in the vehicle now. Even when built right, with the best parts, a 700 can only DREAM of being half as strong as a TH-400.
You say you are not going to be towing.. Let's ask another question. How large are the tires on this truck? If they are larger than stock, have the gears in BOTH axles been changed to match the over-size tires? If it has O/S tires, and the gears have not been changed to match the tires you WILL NOT get a 700 to last in that vehicle. Even if you never tow with it. Period.
My advise-stay with what you have! Otherwise, plan on laying out a BUNCH of cash to get a 700 in the vehicle, and then plan on rebuilding that 700 every couple years. The recipe for a long lasting 700 are simple-light weight vehicles, low horse power, no towing, no over-size tires. If you stick to that you can expect 150K miles out of a properly built 700. If you break any single one of those mandates you can expect to soon be a 700 guru after you have pulled it, disassembled it, rebuilt, & reinstalled it multiple multiple multiple multiple multiple multiple times. Are you getting the picture yet? Keep the 400.