considering a 72 scamp

Hello forum folk,

I'm the current owner/ restorer of a 79' jeep cj7 and chevy van. I'm considering the purchase of a 72 plymouth scamp from an associate to replace the van as a daily driver. this is a rust-free car. interior in great shape. but bunches of shallow dents from god-knows-what. its got the slant six with a 3spd toqueflight. I'm wondering what kind of mileage i can expect on the highway if i drive it conservatively. I know this is a loaded question, but i'm considering it for a weekend cruiser to the lake and family outside the city and want to know if its gonna be a gas hog. Also, has anyone swapped an overdrive unit into a valiant of this vintage?

Cheers.