It depends again on what the ultimate goal is. If he wants something that he can drive around, is streetable, and controlled by the factory PCM, then built 4L65E or 4L80E.
Who cares how much marginally cheaper the TH400 is. Then you add the fabrication requirements, another shifter, plus the fact you lose true automatic shifting since now it's a manual valve body. Want a lock-up converter while racing? Add the additional support and the fact you have to flip a switch while racing. Want that nice highway cruising at a decent RPM? Go look up the cost of a Gear Vendors overdrive unit, that will put him in the range of a build 4LxxE.
Steve doesn't like his TH400 setup and wants to go back to a T-56. I don't blame him since his highways RPMs are ridiculous and his tranny temps scare me.
So factor in Brian shipping his tranny out for a build that costs $2400 versus a TH400, the conversion parts needed to make it work, and the overall hassle/ Yeah, I think spending a bit more is worth it.
But it doesn't matter, let him decide.