I replaced my TH350 with a 700R4 about 6 months ago. I am extremely happy with the 700R4. The first gear is nice and low which is a lot of fun, and overdrive really makes my truck run smooth on the highway. I went from about 12 MPG to about 16 MPG. I have a 3.73 differential, which I think is ideal for a 700R4.
I'd recommend taking a look at this FAQ thread:
http://67-72chevytrucks.com/vboard/s...d.php3?t=33266