"...Then the Turbo 400 tranny will get smoked 'cause it is in the middle of your latest upgrades..."
"...I also had the TH 400 taken out and rebuilt and reinforced to handle the 400 plus HP as well. Again this was done by a reputable local guy, that also said he would stand behind it as well. I trust both of these guys..."
If he installed a 34 or 36 element intermediate sprag, it will probably hold up. If he didn't, it probably won't, if your engine will make 500ft lbs of torque and you get good traction.
My 455 bracket engines broke 2 of the 16 element sprags, one in a TCI TH400, which was supposed to be a "full competition" model with a manual reverse pattern valve body. These engines probably didn't have much over 400hp, if that much, but they probably did have at least 500 torque at just over 3000 rpm. Replaced the broken sprags with the 34 element sprags, which, at that time I think my trans guy said was a GM part which originally came in some HD TH400's, like maybe the 500ci Caddy models. Never broke one of those in our low 12 & high 11 sec bracket cars. Hundreds of runs.
So, if it was me, I'd ask the trans guy if he used the 34 element sprag or not. If not, I'd have him go back in and do it. If it has the later model drum which had the weak roller clutch instead of the 16 element sprag, he'll have to change out the entire drum/sprag assembly. But, they are readily available. Lots of 'em on Ebay. Either the early TH400 or the later 4L80E drums will work. Ebay always has quite a few for sale, bare & complete.
https://www.ebay.com/sch/i.html?_fro...ag&_sacat=6028