I would say getting this small bearing 9" is going to be a bigger hassle than what it's worth, but that's my opinion.
I can't think of a donor Ford with a 31 spline 5 x 4.5 bolt shaft for the 9", unless it's custom made.
IF there is a donor car, seriously doubting it though, the shaft would probably be out of a EB or early F-150. Maybe from the 60's or early 70's. The width would also be a problem too.
For gearing, the best way is to take it the 3rd member out. It's also the best axle to learn how to do gearing, but I wouldn't recommend taking on that challenge unless you have someone who knows what they're doing to help you. The cost of gears vary, you'll need to research that one. Take a look at
www.completeoffroad.com. They also sell stuff on ebay. I lucked out and got my 4:56's with a full Detriot for $350, it pays to know certain people!!
I would suggest going with a big bearing 9" out of a full size bronco or f-150, the only problem is that they are 65" wms-wms and a 5 on 5.5" bolt pattern. It sounds like you're wanting to keep your existing wheels and the keep the same width as the front. Am I correct?
All 9" pinions are lower than a Dana or Yota axle, stock 9" axles that is. The custom Currie, etc... are the only high ones. I've been meaning to get some pictures of a 9" pinion height, compared to other axles with the same set up tires. Then I'll have some real physical proof of whether or not there is a big difference in pinion height.
If you are intent on doing a 31 spline conversion, you'll most likely have to have custom shafts made. But I've got a 31 spline open carrier that you can have. The carriers for the 9" don't have break points, which I think is a plus. So you can just go to a yard and pull any 9" carrier and use it. I've got the old 3.55 gears too, you'd just have to pick them up or pay for shipping.
Anyway, that's my opinion. Also try looking at the FORD section of the POR board, just becareful of posting over there. As a general rule for the POR board, "research, make sure NO ONE has asked that question before, and don't ask dumb questions", you'll get your ass chewed.