I hate to sound like a total a$$hat, but I think what you are after is out of your price range. It's the good mileage part that tips the measures because to get that you need diesel power and to get diesel power you are going to have to come up with a big bag of money.
IIRC, you were looking at an '89 Cummins for $6K. If that is your price ceiling than you *might* be better off sticking with your F350.
Forget those 89,90-91 Cummins Rams IMHO. In stock form your F350 460ci will likely out pull one of those early trucks, or at least stay right with it.
Like I said, I'm not trying to be a jerk by basically saying "you can't afford what you are wanting," but I'm just trying to be realistic.
The way I see it you have THREE options.
1. Just be happy with the F350 and run it, if all you do is tow with it, gas doesn't cost that much when you factor the actuall mileage the vehicle sees.
2. Hunt.... just wait for the right deal to come along. Eventually a 93-98 Ram Cummins or 95-97 Ford F350 will come along under priced and you could afford to snatch it up.
3. Take your $6K or less and make a DOWN PAYMENT!

If you could afford a medium sized payment like $300 per month or less, you could get a pretty decent truck. A 94-98 Ram Cummins in the 10-12K range and even a 95-97 F350 for just a tad cheaper.