At last, I am not alone. I am doing a similar(kind of) swap/mod. I am using a 97 f350 4x4 chassis, 1970 Chevy passenger car 427, NV4500, 1970 chevy body. I have the 97 stripped and am rebuilding the dana 60's and prepping the frame now. Have the 427 and nv4500 ready to go in. Will be mocking up the cab and engine at the same time. I have a 71 k20 bare frame for all the dimensions. Just get both of your frames on a level floor a plum-bob, tape and chalk and go at it.
I am also building a modernized 79 F250 4x4. Well it is a 79 4x4 frame with a 73 F100 body and an 87 4.9 with the trans from the 97.
The 97 will be a toy and the 79 will replace my 99 Tuscany as my daily driver.
Glad I am not alone on this.