A 4000 lb pickup and a 100 hp engine makes for a slow ride.
I don't mind the idea of a six, but the long stroke 225 sounds thrashy to me, I don't care what exhaust one runs. Little pistons traveling way too far per revolution.
My only slant six experience was my grandpa's '64 Valiant. After he died we got to drive Grandma on long trips to see her other grandchildren. I don't remember getting over 23 mpg at 70. Long trips on the Interstate were murder. That constant commotion from under the hood drove me nuts. This was on a properly-maintained car with 65,000 miles on it.
Also consider that it's a 4 main bearing engine. While that may have been enough in the '30s, 7 main bearings make a much stouter bottom end for turbocharging, and that is how I see a slant six being useful. Throw about 10 pounds of boost on it and suddenly you don't have to rev the @#4% out of it to get somewhere. The manifolds are both on the same side and the slant configuration makes for a neat setup. But there's no way it would be less expensive than a $100 roller cam 318 from a Fifth Avenue, and would probably get the same mileage.

R.