First gear woes Options

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Ok guys - lets see if we can figure this one out (please). I’m trying to get a healthy/ functional gearbox in the ‘73 1.7 car.

This is the car that had the bearing whine but otherwise shifted well. The whine was loud and getting worse so I pulled the box. Other than the whine the gearbox shifted well and would hold each gear properly. Let’s call this gearbox A.

I replaced that gearbox with one that I bought from a local member that was known to be good except for popping out of first gear. The first gear dog teeth on this one were nearly totally gone. The box looked good otherwise. I ended up re-using the same first gear shift fork as was in this gearbox which may have been a mistake). Let’s call this one gearbox B.

When I removed gearbox A from the car I opened it up and harvested the complete first gear cluster but not the shifter fork. I then installed the first gear cluster into gearbox B and then put gearbox B back into the car.

The car is still popping out of first gear - which is a bummer. It will go into first gear but as soon as the clutch is gently released it pops out. You can see the shift lever actually move forward when the clutch is let out.

Shift linkage cone screws are tight. I’ve adjusted the shifter and can get reverse and 2-3-4-5 cleanly. It holds all the other gears properly.

What I am wondering is if I have a bad shirt fork would cause this behavior. I am also wondering if I may have mis-positioned the shifter fork on the shaft. OR do you think I just have an incorrectly aligned shifter? I suppose I can disconnect the linkage and manually put it in first and see what happens.

Any ideas for me? Should I open it back up and swap in the fork that was in gearbox A?

Picture; Not my gearbox but this is the depth question I have - how far down on he rod should the fork go? Mine is slightly lower on the shaft than this.

Thanks!