Vacuum on 1968 GTS 340 4 spd.

Well, that is all good on the gauges! It really can be misleading with bad tools.

Hate to say this but your bobweight is now off by 150-200 grams with stock rods depending on the exact piston..... dang. Drop pan and timing cover, pull crank, and rebalance crank alone to a new computed bobweight would be the easiest way, and would be a decently good balance job. The piston/rod assemblies are going to be close to each other since the KB pistons are already closely matched.