take off the vac adv hose and cap it. Make a mark on your harmonic dampener 2&1/4" (which is 35.5 degrees) to the right (clockwise) from the factory slit. Run the RPM up until it wont advance anymore (note the rpm) and lock the dist down w it timed at your new mark. Bring it back to idle and note your initial (post it). you stated that your eng functions well @15 initial (vac dis/plugged) so if this new total mark puts your initial way off from 15 you will need to change the slot lengths so both your initial and your total are pretty much spot on. THEN hookup your vac adv to the pass (ported) nipple and turning it inside w a 3/32" allen wrench CCW will slow the onset of the beginning of the curve by making it take more vacuum to offset the spring tension in the vac can and may need a can w less total (5-9 stamped on the arm works well which is 10-18 added at the crank). put the vac gauge on the dr side and adj for best vacuum on the idle mixture screws then lean them a bit then reset your idle speed then redo the mixture adjustments. Not sure how far to turn each mixture screw in to lean each one after you get the highest vac??. And what springs are in the dist as ideally you would adj: initial then the total then the springs picking a sp0ring combo that will keep you just under the pinging point @WOT then adj the vac can staying just under the pinging point @ PART STEADY THROTTLE at an RPM where the springs are maxed out. Still a prob check rotor phasing by drilling a 1/2" hole in the top flat of the dist cap 2/3 of the way between the center terminal and the #1 terminal closer to the #1 terminal and see how far from the underside of the #1 cap terminal the rotor is at the speed where it runs ragged by freezing the rotor w your light.