Have the valve springs been checked? Just because the spec says 150 lbs. on the seat doesn't mean that that's what they are. You should have at least 140 lbs. on the seat. If the pressure is just low enough it won't sufficiently control the valve. I saw it on a friends 440 that out of no where slowed from 11.20's to 11.60's and the mph was down also. He checked the springs on the car they were all around 120-125 on the seat (very similar cam specs.). Installed new spring's @ 140 on the seat et and mph came right back. I also think pulling the heads to change to a steel shim gasket is a good idea but while the heads are off why not have the heads cut to bring the cc's closer to 78-80 cc and the springs can be checked at the same time.
According to the Kieth Black compression ratio calculator;
80cc chamber
4cc for valve relief's
.021 gasket
4.5 gasket bore
4.35 bore
.015 deck
3.75 stroke
= 10.81 compression
If you cut the heads and or change the gaskets don't forget to check piston to valve clearence.