the more I think about this engine the more I think any kind of performance ignition curve is a waste. your engine, in my opinion, is more efficient than a stocker with that high vacuum. I have a stock 440 I run around in and it's not as efficient as yours. i'd 'bet it's doing well over 20"hg vacuum at cruise speed; maybe 22-24",...? i'd try a curve very similar to the original mopar performance electronic distributors. I think that would be a good baseline and go from there. hopefully your not using a loose torque converter (if this is an automatic transmission) or low rear gears (numerically high).