Transmission info for my 6 cyl. (250)

[GreenBay1921]

I'm working on a '68 C10 with a inline 6 (250) with manual transmission. The truck has been sitting for 23 years. It is making sort of a scrubbing noise, but if I pushed the clutch in it would quiet down. It also made sort of a roaring noise in 2nd gear. I checked the tranny dope & it looked like junk, so I pulled the bottom plug to drain it & 8-10 oz of water came out. I filled it back up with fresh gear oil, & it quieted down some. I pulled the tranny tonight. When I pulled the driveshaft a little more water poured out. The gear oil looks like crap still, so my question is: should I flush out the tranny & fill it up with clean oil, or should I just have the tranny rebuilt? Also, does anyone know what model tranny I have. I'd like to do some online research, if possible. I have this crazy thought that I might be able to rebuild it myself, but I may just take it to a friend that rebuild them instead. Clutch looks good, but the pressure plate & flywheel has some pitting, so I'm going to take care of that as well.