The 66 should have an 8.2" rear; the 7.5" was last used in early 64's.
If the Saginaw is a good one to start with, it should hold up. I beat the heck out of mine and never had a trans problem; I didn't run slicks on the 66 but it did have 235-15 drag radials.
The only way to truly get a good answer regarding how the car will run is figure out what rear tire diameter will be used and figure out where the 230 makes it power. Then see if the RPM range matches the trans ratios. Patick's catalog had a great chart on Saginaw gear ratios and where the rpm ranges fell; don't know if his current catalog does...
