I'd go with the plugs and heat range that those heads would normally have in them in the donor car as a start. Gas mileage: Too damned factors involved even with a fairly simple engine. Cam, intake, carb (s) exhaust. Gears or final drive ratio. Attitude, "It's a hot rod I gotta get on it all the time". Proof in the pudding was back in 1969 my buddy Danny Gonzales got out of the army in Fort Hood Texas and took off to La Putente Ca where he was from as I was headed to Santa Monica and National City and Chula Vista to see family. Each time I drove I pulled 16 mpg and each time he drove he pulled 18 mpg in my car. Just a difference in driving habits. Every combination has it's happy spot speed wise to pull the best average gas mileage. A few years back on a road trip to Kansas and Texas with a meter on the dash that read out gas mileage we figured that running 74 on the80 mph speed limit roads was worth 5 mpg. Get over 78 in that car and the mileage dropped like a rock. Still with a fresh car you have to play with the tune and that might include timing and jetting.
AC R45TS for that. You can cross reference that number if you want another brand. I would recommend NOT Champions, use ANY other brand but them. Doesn't matter what the engine's cubic inch is.
Should be noted, I’d doubt the heat range of the plugs is your issue with MPG. Unless really fouled, something else is the issue.
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