factory rears ends in the IIs are usually 8" though I have seen one with an oddball 7.x" rear at one time. Good gears for the street are pretty much determined by you transmission and tire size, with a good 15" wheel with a moderately sized tire and a non-overdriven transmission I'd go no taller than 3.73s, if you go with an overdrive you could go as high as 4.10s.
__________________ 1976 Ford Mustang II Ghia: 302 with a 600cfm Edelbrock carb, Edelbrock Performer 289 intake, Dynomax Blackjack headers, 2.5" exhaust with Flowmaster Super 44s. RJS 11-gallon fuel cell, C4 tranny, chrome 16" pony wheels, fuzzy dice, brown vinyl half-top, and painted in the tackiest color ever (harvest gold, that's why I call it "The Goldenrod").
Also have a 2003 Dodge Ram (lightly modded daily driver/tow rig/office/dining room/home away from home/workshop... I call it "The Big Blue Dawg".) | 
76 mustang 11 gears!!?? 
