1965 Merc 39 WOn't shift out of forward

[dafaddah]

I just finished replacing the impeller on a Merc 39 I bought used. It's the second time I've had the LU off because the first time there was a piece of the old impeller in the intake pipe that wound up in the pump and partially blocked the outlet. I had some trouble reinstalling things the first time because I didn't have the gear shif in forward. This time i was very careful to take down the LU with the shift set in forward and not to touch the shaft in the LU. Everything went together smoothly but this time the motor was locked in forward. I dropped the LU again.
I can twist the shaft to change gears while the LU is off and with the shaft set in forward and the upper shift lever in forward the LU slips in easily but with things buttoned up the motor is locked in forward regardless of how I move the shift lever. The only thing I can guess is that the spline socket is stripped.but the LU won't join up if the shift isn't in forward so that doesn't seem to be the problem.
Has anyone any ideas what might be going on? Can the shaft from the top end spin in the cam?

 

[dafaddah]

I discovered the problem and cured it shortly after I wrote this post. It turns out that the shift control shaft from the top unit has enough play so that the shift shaft from the LU can actually slip past the end of the socket on the shift control shaft and the LU can be installed without mating the two parts of the gear shift shaft. By using a fine screwdriver inserted just above the lower mount assembly I was able to maneuver the shift control shaft so that the end of the shaft from the LU entered the socket. The gear shifting now operates as designed.