the Promaxx heads have nice finish work.
you should not need special headers with the Indy LA-X heads, do they still make those heads though?

if yr MPH and RPM's are correct, and maybe your tire is more like 30.5", seems you have a 3.55 gear ratio.

I have a W200 with 318/auto, 34" tires, full time 4X4, it's best cruising 55-60mph because 4.10 gear ratio. Straight highway driving it's around 13-14mpg.
stock 318 motor but added headers, dual exhaust, weiand dual plane intake, holley 650, stock distributor with an MSD box.

spend some time thinking about the cam if you want good mileage, around 215* @ .050" for duration and 112-114 LSA is probably what you want.