I have been running a stockish LA 318 with stock electronic ignition w/vacuum advance in my '82 Plymouth Gran Fury since forever; I'm putting a hotter rebuilt 318 with KB pistons and magnum heads for about 9.7 compression (quench engine)- how do I go about re-curving the timing for this new engine? I know with the closed chamber magnum heads I need to reduce the timing advance, but is there a specific way to do it properly? Any guide as to what the new timing specs should be? Do I simply reduce the initial timing by a certain amount?

If you can find a copy this is the ticket

The Doctor's Step By Step Guide to Optimizing Your Ignition

I have a copy here and it' pretty useful for what you want.

set the initial with the "vac gauge method". If the starter kicks back when hot, reduce the initial amt (1) deg & retry OR keep it where it is at & wire in a momentary NC contact switch & push it then start cranking then release it while you are cranking. Purchase the FBO $22 plate & select the slots to get 31 total (initial+slots). toss the heavy spring with the elongated loop on one end & sub in a mr gasket 925B or transdapt #4450 or a MP light spring in its place for a baseline start. dial in the springs then dial in the can.

OK Vac Gauge method is simply dialing in the initial to make max vac. hot idling in neutral correct?

On your deal I would shoot for between 14 to 18 degree BTDC on the idle timing and use softer springs to speed up the mechanical advance so it is all in at between 2000 and 2500 RPM. You will probably need to shorten up the advance slots ( I weld them up)in the distributor so you get between 4 to 8 degrees distributor mechanical advance (which is 8 to 16 crankshaft degrees). I have not work on or raced a motor exactly like yours but all the iron headed LA Mopar SB I've work on and tuned like 32 degrees total advance at WOT above 2000 RPM It is far better to start out with less total timing than to much total timing

in gear (auto) with ebrake on (which is what the eng will see/be at stoplight to stoplight) or neutral for a stick, keep advancing the timing till you reach max vacuum (all the while constantly reducing the idle speed back to its lowest normal speed that it will function at), then turn the dist housing back the other way till the VACUUM drops exactly (1) in hg & readjust the idle speed if needed & at that point your INITIAL amount is set/correct. & you want the dist springs to start advancing no less that 1-200 RPM above your idle speed, so if you idle at 800 you want the springs to start advancing at ~1000. higher is OK (but hurts performance/economy/runs hotter) but not less than 1000.

btt