I did mine pretty much the same way, including using a die grinder disk on a pneumatic tool to cut the lower bracket. It's such a clean swap that I couldn't understand why anyone would even consider keeping the Lucas junk. It literally and figuratively is a "day and night difference" in the electrical system capabilities. I also like being able to see.
I didn't have to offset mine on the lower bracket, as it appears your casing is slightly different than than mine. The lower bolt fits right through the alternator casing and through the lower mounting bracket.
If you buy one from the local Joe's, just tell them '78 Camaro 305/350." Any late 60's/70 GM alternator on a 6 or V8 car- they are all standardized Delco's of the era.
Those Mr. Gasket packaged universal alternator brackets pretty much grow on trees. You can find them in the "Blingware section" of any corporate auto parts store.
I bought a 63 amp Delco on Ebay that was already modded or a one wire regulator for $70- which is roughly the same price the local Joe's sell an alternator for, plus adding the purchase of a one wire alternator (I really wanted to do it one wire). I had to "clock" mine also- be careful as it's real easy to break the porcelain regulator upon reassembly, should the regulator parts come exploding out of the casing.