I am rereading this thread as I want to change the mechanical advance on my 340 with Edelbrock heads. I have the original Mopar electronic distributor with the slotted t-bar advance mechanism and have to weld or limit the travel of the advance plate. What I need is a guide for the length of the slot vs the number of degrees advance in the distributor. Has anyone done this measurement?
BTW, the vacuum advance only comes into play during cruise when vacuum is high. Vacuum advance doesn't work at idle if you use the ported slot in the carburetor. Vacuum advance is important for fuel economy and proper combustion at cruise conditions. You generally want about 50° total advance (maybe more) to optimize combustion and fuel economy, and prevent fuel washing of the cylinder bore which reduces engine life. See a number of articles in Mopar Action for details. Properly set up, my 340 got 19 MPG on a cruise of 60 miles. The combination at the time was stock 340 heads matched to the intake (AirGap) and throat ported. 284/484 cam, A727, 3.55 8 3/4 rear, 245-60x15 BFGs.