Was the company you purchased the tranny from installing new cases or just welding the broken one?
Swiley383 is correct referring to misalignment or a bent component. I had a case with a misalignment and
it cracked the outer case. No matter what I did I couldn't make it work to my liking and finally replaced the case.
As far as what trans is correct for you? In a 3700 pound car, with 580hp I most definitely stick to a 3 speed.
A TH400 is built for heavier cars. If you decide to up the HP the TH400 can handle everything you can throw
at it. There are some good high-performance transmission shops out there and building a TH400 to handle 2500hp+
isn't unheard of. Good luck!