I have a '95 5.8L (351W). It is the highest evolution of the motor with roller lifters.
If your motor is healthy as-is, be sure you have a good rear gear ratio first, along with freed up exhaust...as much as you can get away with in California. Mine came with 3.55 and Track-Lok, and that was the best ratio offered from the factory in 1995. It is a relative dog towing even in mild hills. I'm going to 4.30 with my now 31.5" tire and wheel upgrade, and even that is a slight compromise for my occasional towing use, with 4.56 being ideal with heavier/frequent towing.
You didn't mention towing, but our motors generate max torque around 2,800 rpm and max HP approaching 3,500 rpms. With an E4OD in drive (OD locked out) if you aren't getting above 2,800 at highway speed, then you are missing out because the Windsor is just getting ready to work at those rpms.
Conversely, if you end up in 2nd gear and running around 4K on a long grade, you are past the power curve and not getting any extra for your efforts. I've learned to accept that our lowly 5.8L engines are a dog compared to modern V8 injection technologies, but we have an advantage of easy maintenance and legendary reliability!
Free up your intake snout as well, remove the noise cone, and cut away the block on/in the air filter box if you haven't already.
If we try to build power in the engine using traditional "hot rod" techniques, we can move the rpm scale up too high to do us any good with a heavy old van. You mentioned mods that worked with stock ECU so I guess you've taken that into consideration.
I've learned to adjust my thinking a bit to match the motor capability, leave a little earlier, and enjoy the ride within the limits of the good ol 5.8L.
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