Howdy Back Nick:
The 4% increase/decrease is related to a stock engine with a stock cam. When you add a performance cam with more duration it, in effect decreases the dynamic compression, effecting low rpm performance. By increasing the static compression some of the dynamic compression is regained. So, in your case the 4% +/- will not apply.
That 4% figure is somewhat of a generality. Few, if any, will only increase the CR. It is increased along with other performance enhancements at the same time. Also few, if any, will invest in a before/after dyno test to empirically test. The 4%, IIRC, came from some baseline testing on a small block GM V8, in the Comp Cam dyno room. So, for our purposes, consider it a generality at best.
Is your distributor mechanical advance curve still stock? If so an advance recurve would also help. The DS II distributors stock advance curve is very slow making for lazy performance. That, and your cam, is why more initial advance is recommended. Did you get any knock at 15 BTC?
The sidewall rating of your tires is Max Load @ 35psi. You can safely go to 40 psi, but watch for unusual tread wear in the center.
Other vehicles (Mavericks, Comets and early FOX based Fords) used the 7.25 differentials also. 60s came with 3.20:1 in Mustangs with 200 3 spds. There was also a 3.25:1 in Rancheros. They are a code 4 or 5 on the door tag under "ax". Late 60s and 70s are coded as follows 3.20, 3.25:1, 3 and 4, open and limit slip. Most of these are 4 lug, drum brakes and the correct width. Keep an eye out for one of your V8 buddies who are swapping in an 8" or 9" rearend.
Lower gears, looser converter and smaller tires are your best bet to spinning your tires- but it all helps.
Adios, David