Cool to have another build to watch. I can't recall, is the TH400 longer than a TH350? Did you K5 have a V8 before or a 6 cylinder? Use the V8 motor mount position and everything should fit right in. Will you be doing anything to get your rear axle centered back in the wheel wheel? The shackle flip pushes the rear axle forward a bit as you see.
I also used 78 3/4 ton gear running gear in my 70 K5. Have you hooked up your front brakes yet? I added the 78's master cylinder to my K5 to get power brakes and ended up doing new hard front brake lines - otherwise I was going to have to have adapters at the master and also where the hard lines meet the rubber front lines.
Welcome to the board and again, will be fun to watch your project.