I'm a bit of a mopar slant six guy as well. (I mod on slantsix dot org.)
What percentage would you say is in town driving?
In the 80 you should have a Carter BBD, though some still came with Holley 1 bbl carbs.
Does it have a lean burn system? They were, sadly, terrible.
The rod ratio of the 225 is, for lack of a better term, dismal. A 6.7" long rod for a 4.125" stroke.
Improving the rod ratio is easily done by going to 198 rods, (7.005",) or an aftermarket 7.0" rod then over-boring for a piston with a lower pin height. There are many options. It's easy enough to do, but requires a pretty heavy financial outlay. Installing 198 rods with Neon pistons nets zero deck height. The stock engine configuration leaves the piston at -0.140" or more in the deck. Stock comression ratios run the gammut from 7.5:1 to 8.0:1. No rhyme, not reason, though the trucks tended to get the short end of the stick. We've seen stockers approaching -0.180" down the hole.
My Primary engine runs a 4.440" Stroke, 198 rods, and 3.445" Bore Toyota pistons. (I run the 4 banger pistons, but with a bit less stroke, the '88-94 3.0L V6 pistons from the pickup could be used. The small end of the rod needs to be bushed to fit.) My pistons are 0.070" Down the hole, with 0.125" off the deck. Couple that to 0.040" off the head, and we get 10.3:1 compression.
With an RDP RV cam, engine speed is limited, similar to the stock Marine cam at somewhere in the 4100-4500 rpm range. RDP cams are designed to work with six or four cylinder engines to reduce pumping losses. The intake duration is longer than the exhaust event. Most cam manufacturers utilize v8 profiles that generally have equal duration for both intake and exhaust.
A V8 has 90° of crank rotation between power impulses. A 6 cylinder has 120° between. That means there is a lot longer rotational distance traveled before a power pulse picks up the rotational mass again.
For a 4 banger, it's 180° My Metro's three cylinder runs 240° between power impules.
See where I'm going with this? The engine isn't producing power for the time between those power events but for a short time. The rotation that actually uses the pulse is in the 90° or less neighborhood. Once the crank has rotated 90° past TDC, the force being applied to the crank is drastically diminished, and the engines rotating mass has to carry us to the next brief moment of power. That's why eight or more cylinders can make so much more HP.
The cam choice for maximum economy needs to take this fundamental difference in design into account. The exhaust profile on my engine is basically stock 1974, while the intake has a longer duration event, with no increase in net valve lift.
The ignition curve that is in your truck is not optimized for fuel economy. If you are running the lean burn, there is little you can do with it. Most folks pitch them and install earlier distributors, and various bits and pieces. The GM HEI being the best bang for the buck.
With the earlier electronic ignition from say a 1974, you can change the governer to modify mechanical advance, and you can change springs to modify when the advance comes in. You can also change out vacuum advance cans, some of which are adjustable. My maximum economy was with a 9r governer all in by 3400 rpm, and total timing in the 50° range. I wasn't getting all the mechanical due to laws governing speed. Even with short gears, I'd be going 75 to turn that many rpm.
For primarily in town motoring, going to larger diameter tires will have the opposite effect on your mileage than flat freeway hauling would. Going to a lower final ratio will actually improve economy in urban use.
Out on the freeway, I can pull upward of 23 MPG with the old Valiant on my tall gears only if I don't have to pull hills and passes. Since I tow a trailer and have all my camping gear in the car, I leave the short gears in the rear end. (I run a Ford 8", so can change the rear gears in about 40 minutes.)
Since the whole idea is to keep one's foot out of the go pedal, a shorter ratio for short hauling and towing will net better economy.
Unless you are flat landing it for distance on the freeway. If you have to deal with hills, keep your ratio in check. The 225 is an awesome little engine. But it is little.
What carburetor do you have on there? The 2 bbl Carter BBD will do better than the Holley 1945 out on the road. There are a couple of Holley 1920 models out there. The small venturi was on the early 170CI A-Body cars. It will not work well with something like the van. I know your not in a rush, but you will burn your transmission to fine powder running under the converter stall.
The kings of slant six economy were the Feather Duster and Dodge Dart Light. The percentages sold were very low, so your looking at a one year run. 1976. EPA HWY was 37mpg with the package and an A833 manual. Don't hold your breath though, 'cause nobody got that. I have heard plausible reports of 34mpg with the full aluminum sheet metal though. When you think of these cars, think about very poor acceleration. The early A-bodies were actually lighter than these offerings.
With a van, to beat mileage on a well tuned 318 with the slant is possible, but your going to have to build more power in for the bottom end. 2-1/2 inch exhaust is over-kill. If you can, find a Feather Duster head pipe, it is 2-1/4" stainless. They live forever. If it will fit in the van is another question. This is the best bet for a single stock exhaust manifold. Dutra Duals with a 2" Y pipe to a 2-1/4" single back to the muffler, and 1-7/8" tail pipe would be my recommendation. The modified rear keeps carb heat, which is extremely important for economy. The new casting front really breaths. Many folks make thier own.
The fuel delivery of the slant six intake manifold isn't the greatest technology. The center two cylinders run rich, the next two out run perfect, and #1 and #6 run lean. Poor distribution. There are ways to solve this, but none of them are cheap, and many not particularly easy.
I'd be happy to toss ideas around if you like. Getting mileage into the mid-20's is possible, but will require some extensive modification. A van is a brick, weigh's a lot, and the engine is pretty small.
CJ