It's moot now, anyway. I sold the '73 donor truck. A kid up the street wanted it for his first project, and I figured "OK!". So, now I'm on the hunt for a complete truck, prefer '64 - '66, with a title and with or w/o a drivetrain. Now, I have in mind an LSx swap. A plentiful, reasonably-priced drivetrain that I can get 20 mpg out of very easily and have plenty of power.
I do appreciate your inputs towards a body swap. Learned quite a bit! After doing some measuring, the front suspension is indeed a direct swap from the '73- '88 trucks. Plus, I'd had to have drilled a few holes in the frame for the heavier suspension off the '73 to fit the early frame, had I gone that route.