Wolfman, I think he's converting from a Hydramatic 4-spd, not a TH350 (I don't know how the length of those two compare though).
jbgroby, if you're seeking a reputable like new rebuilt trans builder , I like TCI or B&M. If you decide to go 700R4, which I think is a better choice for your application also, give
Bowtie Overdrives a look. You can also look at
Summit to see some reasonable price comparisons.
Having said all that though.. if you've got an adapter that will allow a TH400 to be bolted up to your V6, and the V6 is stock, and you prefer TH400 because you don't need the overdrive because you never exceed 55 mph AND you're concerned about price - why not just install a reputable rebuilder's TH350? Not as strong as TH400, but you do not need a TH400's strength (look at it this way, ALL the automatics in this era's 3/4 ton 4x4s were TH350s...) and it sounds like you don't really need overdrive.
TH350 ($993, rated to 400 hp) is generally several hundred $ less than a comparably built TH400. Just a thought.