Some, if not all, 305s have different mounting bolt spacing on one end of the head. The universal crate engines have two sets of holes on them so that either style manifold can be mounted. You can see the two mounting holes on the left side of this crate engine photo:
http://www.lowcostengines.com/images/10067353.jpg
Make sure you measure the distance between the bolts, and that it matches what you have now.
Slonaker