I have a TH350 with small block 350. My tach indicates 2300-2400 RPM's at about 50-51 MPH on 26" tall tires. Does anyone know how to calculate rear end gears? Thanks.

This calculator shows that 3.42 rear gears will give you 52mph at 2300rpm on 26" tires. 3.73's will hit 50mph at 2400rpm.
You need to get someone to check your mph while you hold the rpm at 2300. If your over 50, you've got 3.42's if your under 50, you've got 3.73's.
Heres the calc. click (http://www.slowgt.com/Calc1.htm#SpdInGear)

Thanks...I felt that the car was up in RPM's while cruising around town, but I did not have a tach until today. I had a feeling they were not 3.31's as I had hoped.

3.31's would have you at 54mph at 2300rpm. Are you sure your speedo is correct? Try to pace a friend in there newer car to check your speedo. Your only talking about 6mph at 2300 between the 3.31's and the 3.73's
3.31= 54@2300
3.42= 52@2300
3.73= 48@2300