Synchro problems on these 'boxes usually show up on 3rd and 4th gears.
Sounds more like a linkage problem and the first suspect has to be the lower engine mount having collapsed and causing the whole engine/gearbox to be misplaced. I've a feeling you've checked this though

Have you checked the gearbox mount? Mounts usually give themselves away by clonking on drive takeup/overrun.
Other than that, check the linkages carefully, especially where the bellcrank balljoints mount on the subframe. On Pugs these work loose and cause all kinds of selection mayhem. A good grease of the linkage balljoints may be in order as well. Snap and balljoints apart to grease them but don't use too much or you'll never get the joints back togeter again due to hydraulic locking.
One long shot is the long rods between the stick and bellcranks may be bent following an accidential grounding. They're quite thin and don't take a lot to bend them.
The actual selection mechanism inside the 'box is of very stout construction and hardly ever gives any problems.