a th350 can be built to put up w/ alot of abuse, but costs.
th400 are very stout in stock form, and can be built to put up w/ just about anything (vast majority of trophy trucks run 400's)
th400 does have one big weakness. there is a position where they will suck air, and will QUICKLY burn themselves up. Its happened several times at rock crawling comps.
For what it sounds like your truck is, its prolly unlikely that this should concern you, and I'd just go straight to the 400.
--B