Newb Transmission Advice

[trailcarnage]

When I purchased my truck a couple of weeks ago, it was advertised as a 454 with 700R4 transmission. With my utter unfamiliarity with anything non-Toyota, I decided to do my own confirmation. After no avail trying to view the casting number I finally borrowed a snake camera. After a bit more messing around, I was able to get a good peek at it...14015445 which comes up as a 75-90 454 for Car & Truck. Good deal! As advertised.

Now onto the primary issue, the transmission slips terribly. I was considering having it rebuilt until I started reading about it's strength behind a BBC. I then stumbled across a posting for a Turbo 400 from a 63-64 Chevy 3/4 ton. Knowing that I am going for a cruiser and not a racer can I get some advice about an auto tyranny selection. Also know that I drive about 40 miles each way to work in the summer so mpg is a slight consideration.