Speaking from experience. Change one thing at a time when you can. However pulling/installing the engine and trans at the same time is much easier.

Enjoy the car for a while between parts. When you change to many parts your bound to wind up with multiple problems.

As for the gears. You need to ask yourself how much traveling are you doing? Will you take it to cruises? Car shows in other states? Drag race? Will you ever put in a overdrive? I would hold on to the gears as thoes gears are becoming more and more in demand.

I have 3:36 gears and I'm thinking of going to 3:08s. I only get 11mpg and the rpm is about 2500 for 60mph. It would be nice to knock that down a bit.