You don't see many 75 Roadrunners. Put 3.55's or 3.73's to give it some oomph. But then you might need to go to a higher stall convertor, I'm not to learned in that area either. Just don't so anything you can't undo, that's a low mileage example. A friend has one with the 400 in it. Was going to sell.it but backed out of the deal, and I'm glad he kept it.