But your cars are no longer stock. So your two setups, especially the 340 with the XE274 cam may not act as AJ described in this statement "Obviously, as soon as the throttle is opened, the vacuum falls and, with it, the low-speed vacuum-assisted timing."
The manifold vacuum from idle (at say 700 - 800 rpm) to off- idle may go up. That would largely due to the valve overlap causing pretty low vacuum at 800.
