I would keep the Pontiac motor if you could. I wish now that I did, but I did mine back in the late 70's
If you do the SBC, you will need to change the starter, and alt. wiring as they are on the opposite sides, change the motor mount frame mounts. and when you do there are two different ones for the small block, just make sure the motor mounts to the motor match the frame mounts. You will also need all the acc. brackets, pulleys, and a chevy starter. as far as the clutch instead of using an adapter just get a chevy bell housing , and clutch linkage. Depending on which brand of headers you use you might have to change the steering linkage. I had to with my headers as the linkage rubbed on my headers.
Everything you need will be for a 69 Camaro, or 68-72 Nova.
So as you can see it is a lot of work and parts to change. It would be cheaper to rebuild your 400 and keep it all Pontiac

If you insist on the SBC you can message me and I will help you with it