Vacuum on 1968 GTS 340 4 spd.

Do you have a PVC valve from another SBM engine? If so, it may be one with a higher transition vacuum level, like 14-15" from a 318. If so, it be wide open at idle instead of closed down to the lower idle flow level, and that will dump extra air into the intake. Pinch down the PCV hose and measure how much the idle vacuum changes.

Also, the secondary throttle stop screw may be too wide open. This can cause low vacuum by 1-2" and cause associated off-idle problems.

Can you explain the "won't go at WOT" comment a bit more? Just dead feeling? Won't rev past *** RPM? Misses and stumbles?

New lifters on the cam or the same ones in the exact same holes?

Premium pump fuel? Which Eddy heads? The one with the 'open chamber milled in them. That is an awful lot of timing to put in for true 10.8 SCR on pump fuel, especially if it comes in early. Just concerned that it is on the edge of detonation.