Dont waste your time adding a leaf or re arcing. The setup you've outlined would swap ends pretty fast in a hard turn. First I'd suggest my steel leaf springs which are a close to zero arc, correct rate spring. Second, either pony up for a bigger t bar or lose the rear bar. Get as close to or the same size tires front and rear. These cars do weird things with mismatched tires. Most like the look of big/little and I get it, but that combined with high ride height really takes its toll on performance at speed.