I put one in my 79 GMC years ago. It ran pretty good back then. I used the following parts: Edelbrock Performer intake, Quadrajet carb, Blackjack headers, Crane Fireball 2 camshaft - no longer made

, hi volume oil pump, double roller timing chain and stock heads.
This combo ran pretty good in my truck. A lot of people thought my truck had a big block but it didn't. I admit it, I ran it pretty hard too. The specs on the cam were a 290 duration and .454 lift. Since this cam is no longer made, I would look for something close. This combo would turn 5000 RPM easy and set you back in the seat. The truck really came alive when I kicked it.

Yeah I had a lot of fun in it.
You want to make sure you have the correct balancer and flex plate for the 400. The 400 is externally balanced so this is critical. I would also suggest installing a shift kit with the off road option in it. I used the stock valve springs as well. If I was going to build a 400 today, I would use a Performer RPM intake with same parts. Good luck with your build. I hope this helps.