Nick,
I can't speak directly to what is going on with your pre B 75, and I'm a relatively new 75 SP-01 owner that has heavily upgraded my gun with CGW parts and kits without any issues.
There are other members that have a lot more experience (and more 75's, 85's, etc.) than me, but there is one member, Scarlett Pistol that posted some excellent 'how to' vids on rebuilding the 75's, if it wasn't for his sear cage rebuild video, I would never have been able to get it back together. If you decide to delve into the sear, I believe the pre B's don't have he firing pin block which means no lifter (or teeny tiny spring) to deal with. For what it's worth, the decocker models are even more complex.
Here is a link to his vids.
http://www.czfirearms.us/index.php?topic=85985.0Good luck.