The 4500 is a direct rail to the shift fork connection where as the 5600 uses a socket that sits between the rail and the fork. So in relation to that with 3rd gear being the farthest gear to reach on the rail for your main shaf it has a little bit longer distance to travel before engaging. Doesn't feel that way but once you open the case you'd see what I was talking about. At any rate the first thing I'd try and the simplest solution would be to remove the tower and make sure you can engage and disengage third gear with your screw driver or 3/8 extension. If you can, blame the shifter, if you can't it could be a rail issue. Verify that when in neutral your gates line up perfect to make a rectangle. If that doesn't happen then it's probably an issue with the rail.