1966 A100 with a Slant gas gussler. HELP!

Fix what you have, tune it up, mileage will improve.
When was the last time it has tuned up, it is around 50 years old.
I bought this van about 6 months ago. Had to lower it, it had a 4" lift on all 4 corners, replace fuel tank, steering was wore out, replaced all the window seals, changed rear gears( which I will change again probably ) been working on the interior, cleaning up the ragged wiring, Now I'm getting into the performance dept. I have not tuned it up, seams to run well other than fuel consumption . tune up and tweeking the holley is next, but long term I'm looking for more performance and a little more MPGs. I have a nice 360 and 727 both fresh rebuilds and 2 more slants just sitting in the garage that i would want to rebuild if I were to use one. The 360 about a 400+ horse that I built for a street car project along with the 727 that has a hemi stall. I would have to recam the 360 and get another converter not to mention every thing else I would need to do to install it in the van. (Don't think I haven't been contemplating that swap!) Guess it all comes down to $$$ spent and what I would end up with, economy , performance and reliability wise. And then still get 10 mpg. LOL wow that was long . hope I didn't bore you. This is what it looked like when I got it. Sky high and you couldn't steer it.Screenshot_2017-08-18-11-35-23.png