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I wouldn't change to a 3.08 gear. You will be burning up clutches with the 3.08 and a 3-speed tranny. Your best bet will probably be to start looking into the aftermarket. The TKO500 or TKO600 5-speeds will probably be your best bet without changing gear ratios. They're actual overdrive trannys.

The Richmond trannys are more of what I would call an underdrive trans. They have a lower first gear with a 1.00:1 5th gear and they recommend a 2.76 or higher rear gear.

Of course there's also the T56 6speeds. You can still pick these up used and just change it over to a mechanical throwout bearing instead of the hydraulic most of them will have. Changing to a hydraulic isn't as tough as it may seem though.

As far as T5 trannys, I'm not real experienced with them, but I know they won't take alot of abuse behind a stout small block. They tend to hold up well behind V6's and stock sb V8's as long as they're not abused too much.