Um... I hate to tell you this but there are Several different 350's made by GM back in the day...

Oldsmobile 350 (same Physical size as the 455 block, different heads & intake)

Chevy 350 (much smaller than a BBC)

Pontiac 350 (not sure how it compared in size to the Poncho 455s)

Buick 350 (again not sure how it compared to the 455 blocks)

If you don't know which one of those engines it is, you will need to find out. I don't think any of them will phyiscally say "350" on the blocks, unless it is a sticker on an aircleaner or valve cover, which you really can't count on anyway...

Do you have a digital camera, take a photo, and i'm sure we can help you figure out what the block is.