Small block gas mileage DD magnum/LA ?

I looked up the elevation for Clinton, Ohio, and WIKI says it's about 950ft. In my previous post I got 5300ft in my head, so I will go back and rework the numbers, no big deal for me. Ok all fixed.

That 318LA is an 8/1 nominal engine. As compared to 9.2Scr for the 5.2Magnum.
For fuel mileage this is a big deal, because the higher compression makes more power everywhere in the rpm band.
That translates to you being able to cruise at a smaller throttle-opening, which usually translates to less fuel being consumed. At 950 ft, your 8/1 LA will already be down on power, so IMO, the LA is gonna need to bring the compression up; Whereas the 5.2 was born at 9.2Scr already. Plus the Magnum has better heads.
On the cheap, meaning not installing hi-compression pistons, to me, the 9.2 Magnum is the clear choice.

Furthermore;
With that A500 comes a new set of gears, loc-up, and an overdrive. The ratios are:
2.74-1.54-1.00-.69od. compared to
2.45-1.45-1.00 in the A904.
The deeper low is 11.8% lower, meaning your first gear will be quite a bit peppier. With this new low gear, you could drop one rear gear size, and not lose any first gear performance,as compared to the 3.55/904 combo, yet this will drop your hi-way rpm about 9% Even before the overdrive goes in. For example from 3.55s down to 3.23s is about 9%, and from 3.23s to 2.94s is also about 9%

Looking at it in another way, in first gear;
3.55 x2.45=8.70 starter gear for the A904, and
3.23 x2.74=8.85 starter gear for the A500. So both about the same.
But on the hiway, in third, with 26.4" tires, before loc-up;
The 3.55/A904 combo might cruise at 65=2940 (*), whereas
the 3.23/A500 combo might cruise at 65=2670 (*), there's that 9% again
(*) is plus 50 to 150 rpm slip, depending on terrain.
After overdrive,in loc-up mode, the A500 looks like;
65=2030 with 3.55s and
65=1840 with 3.23s ..... there is that 9% again

Because the 318LA is rapidly losing power below 2000rpm, the 8/1 318LA is gonna take more throttle opening at 1840, than at 2030, so in all likelihood would NOT Get much better mpgs.

The 9.2Scr 5.2 on the other hand is still very torquey at 1840, and WILL get better mpgs at the lower rpm, with the right tune.