1988 Okanagan Econoline 250 Question

I can tell you things about that generation of vehicles, based on trucks and cars I've owned and raced. First, the 3:54 gear in back is a good compromise for that vehicle, but with the weight and aerodynamics of a large brick, it's going to suffer uphill and against a stiff headwind. I had an 87 pickup with that combo to tow a car trailer with and it did fine on rolling hills and flat roads, but throw a 40 mph head wind and I had to drive in 2nd to stay above 50mph. Never took it to mountains, but I'm pretty sure you got all it has to offer. You can get a little more grunt out of it with a cam upgrade and with a better intake. I owned about 30 Mustangs that ranged from ''86-91 and raced most of them. The intake on a truck and a full sized car is opposite of a Mustang with the location of the throttle body, but if you can find ( not hard a few years ago) an intake to fit a '93 Lightning pickup, it ran a 351W and made about 300 hp, even aftermarket would be good, either the stock Lightning cam or a rv cam from Crane or similar. Stock Ford cams are tuned for emissions not power. You might also check the catalytic converter, I'm not sure if a van has 1 or 2, my pickup had 1 Mustangs had 2. Anyway, a free flowing cata will help along with a free flowing muffler, the stock setup on my pickup was stupid restrictive. Where I live there is no emission inspection or testing and back in those days, we also bypassed the air pump, there were kits you could use for that.
It takes some work to get decent power from that generation of Fords. I think the compression ratio is low too, about 8:1 different heads can bump that up some and still run regular, there will things you can do with the ECU as well, but I haven't kept up on that for a long time, we used chips, then they went to flashing the ECU, but I have no idea what is availed for a truck.