Double check the shift linkage to make sure it's installed correctly with no parts missing. Also check to see if something was over-tightened. Your clutch pedal may need adjusting also. If you have a hydro transmission, you may need to bleed the clutch again. Cable transmission, inspect the cable for any signs of stretching.
When shifting from reverse to 1st, it may help to shift from R-2nd-1st or R-3rd-1st to help align the syncros in the transmission.