With a 4-sp in it now, depending which crossmember you have to hold it, you have more options that if you had the stock light-duty 3-sp. Take a picture of the crossmember and rear mount area from behind and post it.
From what I've seen around here, the S-10 trannies are all but impossible to find. I don't think they were that popular to begin with, and every hot rodder wants one.