Hi, Wardsweb!
Given your other systems (and past equipment's coming and going),
I'm not convinced that even upgrading to S8R (whether 075 or 077
is used), will give you better sound than what you have already.
For just having the experience, it might be fun to assemble parts to do
so (not needing to stay strictly with vintage parts would be less $$),
but it's not clear to me that paying for proper era 375s needed for a
restoration would be anything but a labor of love (might be hard to
get your $$ back out). I don't know if support/bracing is an issue in
going from 1" horn/driver to 2" in regard to reversibility.