The tab would fit in the upper, that’s how it’s designed. The upper is just short is all. I might just cut the horizontal mounting plate and find a way to retain the BCG in the upper with .175” of the plate cut out. The spring will want to come back so the plate will fly out during disassembly if not retained in the upper somehow.
The whole mechanism won’t crater if it’s .175” shorter, the actual moving part has lots of room still for those that are thinking of modifying the BCG assembly. The spring may need shortening since it’ll be compressed more then normal.