The guy that had the fastest Opala black Chevy wagon & record holder was using a Siamese 3 port head. IIRC it ran a 7.8 in the 1/4 mile. @ about 175 MPH His minimum boost was 30+ & a launch & would go up to 40-45+ psi.
We need to figure out his HP, car weighed around 2400-2600 LBS Who wants to do the math?
Those Ebay turbos try & advertise more than what they can actually support. It's just a bit small. I mean if you had the best set-up I cant see the turbo you currently are using making more than 400 HP to the wheels.
I think closer to 350 HP to the wheels, but I could be off.
The turbo you have are advertising HP @ the crankshaft.
Always good to clean up the deck surface. If you put a true straight edge, it is most likely warped a bit anyways.
Tlowes pistons he sells do not sit flush w/the deck IIRC.
Was wondering if you had ordered a J&S or @ least got your concerns answered?