The fuel capacity change (to 148) did not occur until the H model in '76, to my knowledge, although I will be happy to be corrected.
There was a change in back seats from G to H, see this thread:
http://www.337skymaster.com/messages...highlight=seat
Having flown both, I don't think the noise difference is really significant. The pressurized 337s are somewhat quieter, but the normals seem all about the same to me. Never took a meter to it though.
Individual differences between airplanes (rigging, age of engines, etc.) will totally mask out the 4 knot book speed difference. For practical purposes, the speed is the same.
The rest of what you wrote looks right to me.
Kevin