Thinking about swapping to auto trans:

[Greasey Harley]

I was thinking about collecting parts to swap my '68 c10 to Colum shift auto trans. It is currently a small block V8/SM455 truck.
I'll NEVER own another 700r4 and I don't want any electronics in my truck.
This seems to limit my options to th350 and th400 ...I think.
in '68 the only AT option would have been th400, and I just happen to have a good useable unit in my shop. I know the history and trust the trans.
That being said, I would prefer a th350, as I ain't drag racing and they tend to get a little better mileage, and have a slightly better gear spread.

Okay, so finally to my question
Install the good used th400, or get a fresh th350. Advantages, disadvantages?

I got some time here, just thinking out loud.
Interested in input.
-Russ