I don't think your t-5 is a problem but I bet your mileage would improve with a better ratio in the rear, say 3:42, which would still be stout enough for a load of wood or towing, but improve your high speed rpms.
I don't know much about those MAZDA rear ends other than that they were bought by FORD some years back. Whether it'll stand up under your panel depends on what you load into your panel, if you just drive it and put the kids/dog in the back, probably okay, if you are hauling bricks, maybe not.
It'll probably be easier and safter to find a replacement pumkin with better gearing.
