Small block gas mileage DD magnum/LA ?

For the topic at hand, I noticed you said the LA 318 you can get is a 1986 which means it most likely has a roller cam. In that case you could just swap in a small aftermarket roller cam (or stock replacement 5.2 Magnum cam if you don't need mech fuel pump) and reuse the stock lifters if they're in good shape and bolt on a set of Magnum heads. Probably have to get custom-length pushrods but that's something to check anyway for any build that isn't bone-stock IMO. Heck if the stock heads on that 318 are in good shape they should be 302 castings which are already closed-chamber they just don't flow as well as Magnum heads. Probably not very noticeable in a DD with mild cam. And I believe they don't have as much of a tendency to crack between the valve seats like stock Magnum heads do.

In an A-body with an O/D trans (42RH/A-500) and 3.23 gears I'm willing to bet you could break 25 MPG on the highway, possibly more.