18-spline 4-speed trans value?

About a month ago i helped my cousin go pick up a 4-speed box for his 440 '71 Satellite/RR project. He got it for like $300 so we both assumed it was the 23-spline version. Just for kicks one day at his garage i counted the splines on the input shaft and to my surprise I counted 18 which i triple-checked to be sure. This tranny was taken out of a running, driving car and at most just needs a light freshen up. How much do these usually go for? I'm guessing we should just skip putting it in the car altogether and sell it for extra cash for the project as he would like to ultimately put a t56 Magnum in the satellite and we still need to get a bellhousing, clutch assembly and shifter.