Measure first. I decided on the Ranger frame because the mounts are closest to fitting. You can actually use the stock front Ranger mounts with the front 46 cab mounts if you make a little adapter (they are about three inches too low). The rear mounts can be made to fit with the addition of hollow rectangular stock that fits into the 46 cab rails (if you look under the cab you'll know what I'm talking about). I don't know if the S10 frame comes as close.
Even if it is free, if it doesn't fit as easily as the Ranger frame it might cost a lot more in the end. The 98 and newer Ranger frames don't have I beams and are very easy to lower. (Plus I hate to see a prefectly good Ford sittin' on a Cheby frame
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I really need to update the pics in my gallery, but you can see some of the problems you might run into by looking at the pics. The inner fenders were a total pain to fit over the newer frame. The front springs on the new frame sit on top of the frame rails and well into the inner fenders (the s10 frame will have upper a arms that will cause the same problem. Only the long bed Rangers (and I suppose S10's) are long enough to work. The wheel base on mine is three inches shorter than a stock 46.
Having said all of that, I'd totaly do it again. I'd do it differently in a lot of places, but I'd do it again. It's been a fun project, and when I started it, there weren't any like it. The frame swap seems to be a popular idea, but I haven't seen a lot of them finished. Could be I don't get out enough, and it could be that they just never get finished. Mine is still unfinished.
Put pics in your gallery of your progress. I'll want to see how you overcame some of the problems (other people might want to see too).