why don't you have the intake runners coated with the thermal barrier coating. I think calico does it. Would have to research it to find out if that would help with the heak soak.
Just remember with you stall speed Ant. This is a street/show car....4500 gets your tranny hot trying to street drive and shortens the life of it greatly.
Kinda sounds like a pressure issue with the tranny or like you said converter......Funny how this happens now...when I want a race....sucks about the breakage though, good luck with your GM product.
is it the cargo style van or does it have passenger seats? A friend had had a brown cargo style astro van with a 350/350hp crate motor in it with a turbo 400. Only thing in it was the front seats and engine hump......whole rear of it was empty. Used to have some fun in it.