If you already have the R154...Just spend the money to have it rebuilt.
Otherwise, just go find one in good condition.
R154's have the reputation of a pretty solid transmission.
If you look around, you'll see that there are some Supras making some *serious* power, all with a stock R154.
Depending on your power goals...realistically, an R154 will do you just fine.
If you're in pursuit of nearing quadruple digit numbers to the rear wheels, plan for a GM Auto or be ready to fork over thousands to have your R154 'built'.
Jeff
*Edit* No one has mentioned the A340E probably because he didn't mention wanting to go Auto.
But while we're on topic...A *Healthy* A340 that hasn't been beat to shit by a 16 year old F&F enthusiast, with a Trans cooler will work wonders...But doesn't have the power capabilities like an R154 does.
A340 typically starts showing signs of strong wear around 300-350rwhp.
When I was still 7M, I loved my A340E once I bought a B&M Trans cooler...made 320rw/362tq, Daily driven for acouple years without a peep from the transmission.