I would just get a crate tranny from places like bowtie overdrives . lots of guys here have used them in the past .
keep the 700r4 and have the deep first and also OVERDRIVE with todays fuel prices and highway trips .
my buddy recently did a sm465 to nv4500 swap .
he has 4.56 gears / 38x12.50 / 465 he was doing 3200-3300 around 70mph .
now he has droped almost 750 - 800 rpm and LOVES it on the highway rips to play areas around here.
when I redo my k30 plow truck body next year I have a fresh in the shipping crate a 4l80-e for it and the factory 205 adaptor for the 4l80-e

. bye bye th400 non overdrive .