The OM617 has a pretty high RPM range for a Diesel (at least compared to what I'm used to). Mine redlined at 4500 RPM stock and I'm hoping to get 5K out of it. There are TON'S of MPH/RPM calculators on line and I'm sure all are about equal.

Your Ford ought to have a 9" Ford differential and that has a drop-out center section - easy to change gear ratios. I don't have all the available rations committed to memory but there are probably more gear sets for the 9" than any other diff on the planet. Land Speed Racers use them a LOT for just that reason. In fact, it's common to have a couple of carriers set up with different ratios for different purposes. There are a few different series of carriers for this rear so be sue that your gear set matches the carrier style. Any reputable vendor will check on this before selling you a gear set.

The 9" has pretty high internal friction and if you want to reduce that you might want to go with a 8.8 Ford rear as found under 5.0 Mustangs, Crown Vics, and other bigger Ford vehicles. It does NOT have a drop out center section but they're cheap, many have rear disc brakes, and come in a variety of widths so you ought to find something that will work for you. Most 8.8s will need spring perches welded on for your truck but that really isn't too big of a deal.

Good Luck!

Dan