I'm buying a used transmission for a 1990 ford f-150 with a 302 -2 wheel drive.What should I buy?

I currently have a tranni with overdrive no on on shifter.the overdrive stopped working a while ago now I only can drive in first and second.Should I get a C6 or is it a C9 so I don't have to worry about over drive any more.I do a lot of hauling.I just have to get a different yoke and drive shaft.What should I do?