Can't shift into 4th gear., ¿#&@$%! Options

[Air_Cooled_Nut]

Ok, I've been adjusting the shifter in the cabin per the various shop manuals. Currently, I can get every gear except 4th. Even the slightest of changes doesn't seem to work. If I take slightly larger adjustments, my shifting becomes R-1-2-3 or 2-3-4-5. So, yes, 4th is possible and I was able to use 4th for a couple of months prior to this sudden blockage (for lack of a better word). It's as if a rubber bumper was placed in front of the shifter where the 4th gear position should be.

Now, this car is of questionable past and shows signs of neglect. 1-2 gears need to be eased into otherwise they grind. I do have a rebuilt tranny that I kept from my last teener but I really don't want to install it unless I really have to i.e. the failure is in the original tranny. She's still driveable, it would just be nice to have use of 4th gear.

When I dropped the engine/tranny to fix the oil leaks and clean everything, then put it back into the car, I installed new shifter bushings and cone screws, and liberally greased everything. Could it be that I put the engine in slightly askew? Suggestions/help please.