Welcome to the site, Michael:

I think I'm hearing you say that mileage and performance are your goals. The people at Raptor have been building 700R4 transmissions for a long time that routinely handle in excess of 500HP/500 FP of Torque. They're not cheap, but if you truly want to have a performance engine someday, the 700R4 can be made to handle it. Raptor's site is:
http://www.transmissioncenter.net/highperf700r4.htm

Bow Tie Overdrives also sells a complete line of transmissions and kits to take tour truck from a TH350 to a 700R4, 200R4, 4L60E or 4L80E. Their web site is:
http://www.bowtieoverdrives.com/

Take some time to look on the web and study your transmission options. The 700R4 is fairly straight forward to "bolt in" but will require some modifications in placement of cross member and several other things. You'll need some help actually removing and installing the transmission (unless your Hercules) as they're heavy. Us "old guys" use a jack made for transmissions!

After some study, decide how much you want to spend. If you go high and get a transmission that handles a lot more than your current 350 engine produces, that's ok. If you get an inexpensive transmission and grow your engine beyond what the transmission can handle, you'll buy it again!

Have fun and let us know what you come up with. Put a few pictures of your truck out here for us to look at.

Regards,
Glenn