Some vehicles require a different bellhousing from others due to clutch linkage placement, etc. If you're using the 11" clutch, you'll have to use the larger 168-tooth (14" diameter) flywheel, and that wheel won't fit into some of the smaller bellhousings that were designed for the 153-tooth (12-3/4" diameter) wheels. For example, first-gen Camaros used the small bellhousing with casting number that ends in -403, or the large housing with a number that ends in -621. The 11" clutch won't fit with the -403.
Another potential difference in bellhousings is the size of the hole where the transmission front bearing retainer rides. I don't remember the exact dimensions, but there is a small difference which can cause misalignment if the hole is too big.