360 in a 75 Roadrunner

I agree with, leave the rear end alone.

This seems like a low buck hope for big improvement. There are some things you can do that are better bang for your buck but you're starting with 4100 lbs and 180 hp.

Changing gear ratio in trans isn't cheap, nethier is changing the rear end, unless you put in an overdrive, I wouldn't change the gearing.

Headers, cam, valve springs, new timing chain. Cam that is big in th torque department.