1966 A100 with a Slant gas gussler. HELP!

The OD 4 speed behind a 170 slant will get you incredible gas mileage.I used the 170 in my 65 valiant wagon with a straight 4 speed and 323 rear .I get somewhere near 25 MPG when I travel the 400 miles up to my camp in northern NY fully loaded with tools,camping gear,a generator and stuff.The thing is packed and it pulls right up those long hills .I never have to drop below 3rd gear and some of those hills like the big one going thru the high peaks of the Adirondacks go on forever.