I'd agree that 10 mpg or less is low for a stock truck, unless that is city mpg, driving in mountains, or driving with a VERY heavy foot. My 360 Ram always got 12 mpg. It had a rather tired 360, 727 and a 3.21 gear. The 12 mpg most likely could have been improved. I never even bothered to tune it up.

The fuel and ignition systems should be examined carefully. I'd guess the fuel / air mixture is overly rich or there is a problem with the timing not advancing. (Make sure both the mechanical and vacuum advance are functioning properly!)

An overdrive transmission is often worth 1-3 mpg on the highway. It would take a while for the fuel savings to pay for the swap, but the overdrive makes highway cruising MUCH more enjoyable! I'd seriously consider making such a swap just for the better highway manners. Any mileage improvements would be icing on the cake.

I'd give the truck a complete going over before considering an engine swap. (Assuming the existing engine is in good shape.) I'm guessing the low mileage can be significantly improved without resorting to major wrenching.

Swapping from a 360 to a 318 should improve the fuel mileage. If you have to buy the 318. Rebuild the new engine, and swap it into the truck, I'm guessing the truck would have to be driven for many, many miles before any savings would be realized.