I chose the TH400 because I didn't feel like spending $2k-$3k on a marginally capable 200R4 trans. Yes they are going in the 9's, but I don't have the budget to support a 9-second 200R4.
I can put a $800 TH400 in my car that will last me several years. The extra gear just isn't worth the difference in price in my opinion.
If I planned nothing more than a 12-sec street cruiser, then the 200R4 would have stayed in the car. I plan to eventually put mine in the 9's, so it only made sense to do the conversion right off the bat. No sense dumping money into a trans that would eventually need to come out anyway.
I looked at it like this...
Would I consider putting a 200R4 behind a 454 BBC with Nitrous ? Not in a million years.... So why would I put one behind a turbo V-6 that can make the same power ?
I just got done converting mine over... Just waiting for the driveshaft to get built.
It wasn't nearly as bad as I expected. The factory mount can be used if the holes are widened/slotted for the wider TH400 mounting pad. I had to relocate the shifter cable bracket on the pan rail, and fabricate a new pattern in the factory shift gate to accomodate the reverse pattern.
All in all, it was a pretty straightforward swap.