The MPG differance will not be noticed during cruise but it will help city driving MPG where you have to accelerate it over and over, once you are cruising the momentum keeps them going pretty much the same.

I would just start with the SCAT I beams though, they are 595 grams and are a better rod. If you spent 30 minutes a rod X 8 that means you are gonna spend 4 hours (whats your time worth?)plus you should have them resized for a perfectly round bore to reduce friction and you are still gonna have 40+ year old rods that are 50 grams heavier and that 50 grams can be removed from the crank too so you lose over 800 grams from the rotateing assy and get a much better rod.

I think if I was doing it again with no expense spared I would use a custom flat top with no valve releifs, they always had carbon in them when the rest of the piston top was pretty clean, that means the fuel in there was not burning very good. For a more budget oriented deal you could get a replacement cast piston with no valve releifs and mill the block to zero deck it. For a super serious effort get some used NASCAR rods off e-bay as long as you can find with honda rod journals, get your crank turned down to use them and get a custom flat top with no releifs. If you run any kind of piston where narrow rings are optional use them, also use a zero gap ring if possible. Run a magnum head with re-drilled intake bolt pattern so you can use LA intakes and bolt on a offy dual port. For off the shelf headers find the smallest one you can with full equal length tubes or if you want to make or have some made get them with 1.5 inch primaries about 48inches in length and a 2.5 inch collector mandrel bent to some 2.5 in 2.25 out mufflers of you choice and 2.25 dual out lets. For an carb you can not do better than a t-quad, just start leaning it out till it starts running a little rough then richen it up one step, might as well set up the secondaries for max power since you can decide to use them or not. For an ignition run a MSD6 or similar ign with matching coil. Set up your distributer to run 18* initial and 35 total mechanical all in by 2000, vaccume advance will help but you will not need much, look for a can marked around 5 or 8 degrees and adjust the vaccume advance up till it surges/bucks going down the road or pings then back off a hair. Put a crank scraper in and no tray since this motor will not turn over 5000 it will just be pointless and probably actually hold oil on the crank. For a cam you can go a couple different ways, 114 LSA to keep the mix from getting diluted, no more than 200* @ .050 run as much lift as you can get within those perameters, a roller will obviously reduce friction if you can afford it and a solid roller even more but those kind of lobes will be hard to find, if you want to run abnormally high compression like an atkins cycle just retard the cam to keep the dynamic compression low enough.

I think doing all that can give you an easy and honest 5mpg increase over a foctory 318 2bbl.

Above that I think one of the easiest thing to do to increase MPG on the high way is a higher rear gear, my motor felt like it needed another gear by 55 mph because it made so much TQ down low, I was still pulling almost as much vaccume at 70MPH as it did at idle so it could have easily pulled another gear as in an OD gear or something like a 2.20 rear gear (mine was 2.76 and I experimented with a 2.94 for a little while and MPG dropped pretty bad). Some guy will tell you to go big on the cubes so the motor don't have to work so hard and go to a lower gear but that simply is not true or all the 542 hemis with 4.56 gears would get 50 mpg. Build you car like I said (assumeing it is not a barge or truck) and you could get 30+ mpg and have more power than stock.