i ran 7.56:1 on a .63 t34 and noticed little if any difference from stg1 2wd saff cossie on anything other than on boost (obviously!),round town the only major difference was the clutch,the turbo came in within 500rpm of the std t3 as well. so i reckon 7.5 is a good bet really from my own experiences
oh,and yes it could be held flat out in ALL gears! was a harvey gibbs engine.