Milage increase I don't know. However in my 422 ci duster 3500#with manual 5 speed od it would get up to 22 23 mpg on the highyway at 70 75 mph. This is with 34 ° maechanical and 12° vacuum for total of 46° Total timing. This all being very conscious of milage! But even when driving spirited and careful of the law 18 19mpg was the norm. Gears are same as yours and tires are very close 275 60 15 BFG drag T/As. Rpms at 70 is 1700 in OD
I can't give any prior results, as I stuff an OD into the build from the start, but my Daytona 440/6 pack with a Keisler TKO 600 5spd .064 OD, 4:10 Dana 60, 27" tires, will do 20-22 MPG highway on long cross country cruises at 70-75 MPH, on the street where I have a hard time keeping my foot out of the throttle, it'll suck it down to 8-10 MPG
On the HEMI cars with a Keisler TKO 5 spd .64 OD, 3:54 and 4:10 rear gears, avg highway at 70-80 MPH cruising would be 16-18 MPG, again on the street blasting it, mileage could get sucked down to 5-8 MPG
Played around in the past with various OD's in the .80- .70 range, results were often minimal for the effort, you really need to step into the .64-.50 range for the cost of adding an OD to "pay off"
