Late to the party (just saw this).
If you are looking for a long rod that is longer than the GMC, I believe Hudson 308 rods can be an option, if rare.
That is a comparison of a 302 rod and piston, to a more modern piston with what I recall as the Hudson rod.
Gent that was building the engine claimed to remove 1 & 1/4 Lb, per hole (7 & 1/2 Lb total) with these.
I don't know how much boost I would put on them, but I wouldn't want to go over 5 Psi.
