Pontiac engines can be done that way. If 400 heads are on a 350, the compression is dreadfully low unless there are good pistons inside to compensate.
Olds engines can possibly be done that way but there weren't enough 400 olds engines made to make this likely.
Buick engines are in a few different families, and none of them will allow a 350 to have any other heads than a 350 on it. Period.
This is most likely Pontiac (if it's even B-O-P at all) because their engines only had one family and people often mistakenly refer to them as using the same block. Of course, this isn't any more true than chevy 305s and 350s using the same block.
Numbers are nice, but pictures are better for this instance.
If you have a Pontiac V8, you need the motor mounts from a 301 or 265 equipped g-body, available up until 82. You just need the shells for the upper and then you can use the regular chevy style motor mounts on the frame. You can also get those shells from 77-81 firebirds.
Good luck!