XE274H with 18-20 initial timing should pull around 12in of vacuum at 800-850rpm.
Carb? If a carter/ede, make sure the rods are staying down when idling. If they aren't put your fingers on top of them and lightly hold them down, see if it cleans up a but.
That intake... UGH... they should have smelted all those POS and made air gaps out of them! I hate that intake, garbage piece. use them as doorstops before ever putting them on engine. Can you tell my thoughts on the intake.
Get the idle sorted out, then worry about the total number. If you advance the timing a lot, your total might get out of bounds if you try to drive the car.
