trans swap

[Armo67]

I'm still deciding on where I am going to take my upcoming build, and had a few questions on transmission setups.

my 67 c30 has a 350 and a 4 speed manual in it. I'd like to make it an automatic. would I be better of getting a th350 or a th400?

I was also thinking about adding a gear vendors overdrive to whichever one I decide to go with rather than a trans with an OD since I have both the th350 and th400 readily available to me.