I changed my firebird from OHC to a 68 400. I don't know if the mounts on the late model 400 is different. Things that you will have to change:

1 Motor mount brackets are different.
2 Coil springs are different from OHC to V8
3 Fuel line needs to be routed to drivers side.
4 Positive battery cable needs to be routed to drivers side.
5 Other bugs I can't think of right now. Guaranteed.

I'm sure you will be changing transmissions too, which adds a couple more logs to the fire.

In my case, it was alot of work but well worth it in the end.

I kept my OHC. Doubt I use it again, but could put mine to original if desired.