Best Trans for Street for BBC 496 410's Rear
 

Any recommendations for a full Manuel Automatic
Th-350/Th400?
10:1 BBC 496
580 HP 610 TQ
255-260@50 solid roller
410 rear.
In 3700 LB C10 Blazer.
Keeps breaking cases from Hughes and TCI-stock cases.

Is everything correctly lined up? Breaking cases would make me suspect to something not being correctly in line.

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!