The Tremec 600 with a 2.87 first gear + OD would be a nice package and comes with the Hurst short throw shifter. I love that set up for my car. Of course, it's a bit notchier than your T5Z. Not sure on the rev issue.
Your car is very light. This is a comparably strong race built T5 by Gordon Levy Racing, you select the gears, it would rev and would be 40-45lbs lighter than the Tremec 600.
Gordon is very familiar with Cobra builds & racing. It's at least worth a call.
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