How to know what transmissions my truck is compatible with?

How to know what transmissions my truck is compatible with? - 1

Added (1). I have a 1987 dually ford f-350 with a 7.5 L engine, it was originally a work truck with the tool boxes on the back but I took those off so it currently has no bed. I'm going to replace the transmission because it has had some problems. I went to OReilly's and they told me it takes a C6 transmission. But being that it was a work truck, I assume that it was geared very low, I want to put in a much better transmission to keep up with the huge motor but I don't know what its compatible with?

Your assuming is wrong. I think you will find all C6 transmission will be the same gearing, they don't make that many different transmission. The gear ratio of the rear is where you need to go to change anything.

The C6 is a great transmission. Your best bet is to get it rebuilt. JJ is right the Rear Axle gives you the Final Drive Ratio. You have a Sterling 10.25 with 4.10 ratio, if it's the stock unit. It's not cheap to do, but you can get a Ring and Pinion set in the 3s for lower rpms on the highway. But you'd have to drive 10s of Thousands of miles before you make back the cost in fuel savings. And converting it to a 4 or 5 speed Automatic would be much more expensive than that.

How many gears does the C6 have? I haven't driven the truck yet because it has been stuck in gear