A friend of mine & I scrounged some 64 or 68 pin Mac simms a few years ago. We could not get them to work in a couple of GVP cards. (Never heard of a PC 68 pin simm.)
My GVP-M 060 card at work has 4 slots each for GVP & standard 72 pin simms. You can mix and match up to a point IIRC. With all GVP the limit is 64MB. With the 72 pin simms I run 128MB. Works great.
I looked over one hack to convert 73 pin to GVP. IMO only the serious, experienced, hardware hacker or a thirll seaker with deep pockets should try doing it.