The only bbc that has a different dimension is the tall deck big truck engines (366/427) and the performance variants of them. I suppose you could also get in some clearance issues with tall rocker covers, too.
You could always go Gasser style and use fenderwell headers from Speedway.
Oh, and I've been meaning to post this, I found this site that has conversion axles for the '55-'62 diff. They convert to a 30 spline and later 12 bolt side gears. I had heard that the parts swap, but wasn't sure on the axles themselves. Something to keep in mind, you know.
http://www.tomsdifferentials.com/catalog.asp?pg=14
