Hi Doug, from the picture we can see a u-joint tightened on the splines. That's one point of adjustment. There will be at least one more u-joint and support bearing between the transaxle and the shifter assembly.
It's important to know if any of the u-joints or bearings have moved or shift rods bent since the last time the car shifted properly. When the transmission kicks out a gear it's probably not fully engaged.
Put the car up on stands while adjustments are made and be kind to the transmission while rowing through the gears. If any gear doesn't want to just engage adding force will not tell you anything about shifter alignment.
Since the car is up in the air, you can turn the wheels a bit to help the syncros do their job. This would be the time to have a helper (or two) on hand to push the clutch in while changing gears and turning the wheels. If you have an open diff, both rear wheels will have to turn simultaneously in the same direction.
Turning the wheels and depressing the clutch is required to prevent the shift forks, detent springs and syncros from allowing you to find the optimum shifter alignment.
Additional note: the ZF 5DS-25 and RBT are machined to optimum tolerances at racing temperature. So, on driving or adjusting from cold the shifter will feel stiff and gears harder to engage.
The first step is to release the u-joint shown (does not have to come off the splines - just loose), put the transmission in neutral and confirm the shifter no longer moves the input shift rod in and out on the 2-3 gate.
Take a look at how central the shift stick appears in the housing for center and across the gate. The middle of the throw should be equal between neutral and 2 and 3. Position the stick so it looks right then tighten the splined u-joint at the transaxle.
Another note: Middle of the throw and central on the stick is based on available travel in the shifter assembly, therefore in the middle your shift stick may not be "strait up".
Now you can select 2nd, confirm it's fully engaged and the pull the stick back to neutral. If it comes back to the same position, you're good, if it does not, Do Not attempt to select 3rd gear. Something is wrong.
"Something" includes a) the initial resetting not centered on the gate b) shifter was out one spline but worked on loose driveline mounts c) bent intermediate rod c) hanger bearing loose or its support has bent or moved d) something bent worn or misadjusted in the shifter assembly in the cockpit.
Hopefully it works ok, then gently go through the gears in sequence starting with reverse since it's on a dogleg. If there are further adjustments to be made reverse will be difficult or impossible to engage, remembering to turn the wheels as you go from neutral into any gear.
Let us know how it goes...
Cheers
Ian