Now that we have the gear train out, what was causing gears to popout? Having no clue at that time, we kinda poked and looked around, then decided maybe it was the detent mechanism loosening up (the shift rails were now easily activated). So we went and got new detent plugs and springs.
Installed new springs and plugged it back up. We put markings on the shift rail and re-assembled the shift forks back to their original positions, no jig at that time, had no idea one existed/was needed. Stupid n00b us.
Also installed new mounts, 1 front and 2 rear ones.
Buttoned up the transmission and reinstalled everything. Time for the moment of truth, shifted into 4th, wow, it now stays in there, waited.... POP!... COCK!
The car at this time, was also running poorly, it wouldn't hold a steady idle and would randomly die out while running on the road, had to push the car around quite a few times, we thought we must've disturbed or broke something during the period it wasn't running. So we got around to fixing those issues 1st before attending to the transmission again (transmission was pretty much back to square 1, except it would now take a little longer to popout, if you gently goosed it it'll stay in).