360 Tune Up
Wow, that equals a 2.80 final drive ratio and your tires aren’t super tall so cruise rpm has to be on the low-ish rpm side?
18 isn’t to bad. You should be able to squeak out another 1 or maybe 2.
Personally I think your down to fiddling around to see what it likes best on the carb & distributor. 34 + 17 = 51 but I ask if it was measured and verified. Just Incase.
The best I ever did was 21 on my 360. Backed by a 904 & 2.76 gears and a 235/60/15 tire, or 26 inch tire. This was with better gas than I get anywhere at home. Seems when I’d hit the road, out of NY, the fuel quality increased and it shown that n the odometer.
The best mileage on could get with the OE spark box was about 17/18. Adding the MSD 6A added another 1-1.5 mpg. At home, 19-1/2 was as good as it ever got. Absolutely the best.
My compliments to your ride. Very nice, very nice.