Its a long road . . . Lots of different ways to do these swaps and there really isnt a wrong way to do them either. Im 4 years into my restoration/conversion and nearing the end but can be completed much quicker than mine if you have the funds and time. You can leave the th400 in the truck but I'm afraid you will not be happy with its performance. It will not handle the power of the 6bt and (depending on your gearing) wont go very fast since you wont have overdrive. lots of different ways to do your motor mounts, drivetrain, and other little stuff. My best advice is to look up some other chevy conversions in this message board and find out which route you would like to take. good luck!
1986 Chevy 3/4 ton 4x4 under going 12v/nv4500/205 swap