Your TH350 should be well up to the task of pullin' a duece roadster around. Probably more than enough. Most people don't know enough about them and have only heard that a TH400 is a "better" trannies. A TH400 is definitly a stronger trans, but overkill in most applications I see them in. For what you are tryin' to do I would consider an O/D trans as an update first. It would give you an extra gear for highway cruisin' and a little better mileage. A TH700r4, or even a TH200 depending on what you do to the motor. But to the original question, I'd stay with the TH350, should be more than enough for any mild small block