The 'slop' you describe is common when the parts (interlock & yoke) at the bottom of the shifter is worn. The shifter in my '47 was exactly as you describe. I was able to remove the shifter and fabricate shims to tighten the interlock/yoke mechanism up.

The 1947 truck is a one-year only longer torque tube (62.5 in.) and does not have the SM420 transmission. However, the 3-speed floor shift transmission and bell housing in the 1947 will bolt up to a later 216 or 235 engine.

You may well have a 1949 engine, but it sounds like you have the original '47 transmission.
RdsLessTaken/Don

http://s1291.photobucket.com/user/R...rucks/3speedGM37-47_zpscdb719f5.jpg.html

http://s1291.photobucket.com/user/R...%20Trucks/IMG_20385_zps7803e014.jpg.html