Right on easy and cheap fix. I have shattered a th 350 case and was still able to drive it, so don't thin a broken case will stop you in your tracks but, for it to leak as much as you mentioned ot would have to be a major hole.
If the th350 works for you stick with it. I just don't know anyone that can make them live reliably behind big power (500 or more HP) maybe in a light car. The sprags just can't take the abuse.