Daughter needs Gas mileage from 273 commando

I ran a 318 with a stock 340 cam with rhoades lifters with a stock 340 dual plane, Holley 600 vac sec, stock exhaust manifolds, and dual exhaust.

With 2 bbl heads and intake (10.5 pistons) it idled at 24" mercury, but would detonate.

I replaced the heads with 77 360 heads that I got cheap and put a stock 340 square bore intake on it. (to bring the compression down to 9.2 for pump gas). The manifold vacuum was then 22.5" mercury. It could get 17.75 mpg on the highway with a 2.76 gear 7 1/4 axle, however I ended up breaking 3 x 7 1/4 axles in 4 years and then went 8 3/4. It could go when you needed it to, but economy if you drove it right.

Also, put a vacuum gauge in the car and show her how to use the gauge to get better gas mileage.

I would definately keep the D4B intake, and run an electric choke on the carb.