I have a 1970 Camaro with numbers matching 350 engine, and
I have a SBC 400. Would the value of a base 350 car change
much if I swap in the 400? I would understand if it were a
Z28.

Patrick,

I think you will be fine with any driveline swaps as long as you keep the original equipment and make it available to a perspective buyer should you sell the car down the road. Charlie