The TH350, if you can find a decent used one, will be the least expensive way to go. If you are happy with the rpms you are turning at freeway speeds now, you will be happy with TH350. If not, then go with the T5 or a R4 tranny. Any of them will require an adapter for the motor and the R4s will require you to wire up the convertor lock-up. As long as you plan on keeping the 235, any of automatics will be plenty strong enough.
Good luck with your project.
