I just put a 4.3 and th350 in my old cj7 with a painless harness, it was a pos
some leads too long (easy to fix) many too short (not easy), if i had it to do again i would use an oem harness and strip out the stuff you do not need. And not pay the money for the pos.
About Fuel Mileage, I Am Not Sure In The World Of Fuel Injection But In The Old Days We Used Methanol In Snowmobile Racers And We Had To Increase Main Jet Sizes By Triple + That Spells Poor Mileage To Me, Also They Ran So Cold The Pistons Would Not Expand And The Piston Skirts Would Break.
Normally Less Resistance Makes The Flasher Flash Slower, If The Flash Is Real Fast You May Have A Slight Pinch In Wire Insulation Or A Slight Short, If You Can't Find Anything Try A Truck Stop They Have A Motorized Flasher That Flashes Correctly With Any Load