Vacuum advance tuning to maximize cruise MPG?

I'm open to suggestions.
Car runs great and I'm taking a trip this weekend and will see what MPG is with the changes I did today; at least I'll have a baseline to work from.
I recently bought an MSD 8680 from another member and when it arrives I will install it. After install I'll be able to retard and advance the 6AL box while driving and see how the motor reacts. Plus for the 8680 box is I may be able to dial back and run reg or mid grade for hwy cruise.
I looked at the vacuum advance plate today and it looks like I can drill and tap a 10/32" hole and install a screw to limit plate travel. This would allow max degree adjustment stop. The 3/32 allen adjustment in the can would be used to adjust inches of vacuum to move it there.
I took the 3.91s 16 MPG HWY out and installed 3.23s. I drive my car almost everyday and I'm planning a trip this summer to see my grandsons in SD.
In FL you need to run at least 70 on the Interstate or you'll get run over. Car is pretty original and I don't want to cut it up to fit an overdrive tranny.
Gas prices are down but it's only a matter of time before it gets out of sight again. Recent refinery explosion and striking workers and it's already going up.