controlled MPG testing is actually *really* hard to duplicate. I had a 130 mile round trip commute ~3000 miles a month and it was hard to test for MPG changes. Same 4miles in town, 61miles interstate 5 days a week. With careful observation i could throw out outliers i.e. weather and catching too many lights in town, and non commute errands where the biggest MPG killers, but I still would need a 'clean' 2-3 tanks of this very repeatable driving to know how that change i made impacted MPG. But you have a very tall brick, ugh good luck. In the end you only need xHP to maintain highway speed.
tires up to sidewall max cold pressure, good diff oil, good exhaust, non E gas (may not be as big of a deal for carbs IDK) then start there. They make 1 5/8 headers for Bvans. ;-) just saying.
