Another alternative is fit the 850 differential housing to the 4 synchro box.
But it's not always simple because the bearing bores may not align perfectly. They were machined with the gearbox case.
I've done 4 over the years but had to remachine 2 of them to fit. Handy that I have a mill.

Edit
BTW the 850 original shift is the magic wand.
The later one in Tim's pics is affectionately called the puddin' stirrer due to how it rapidly develops slop from new. It's agricultural compared to the proper magic wand.

Essentially you just need to swap the diff cover to an 850 type. You’ll also need the vertical shift rod which has a cup on the end. If you already have an 850 gearbox you could use these parts

They will often bolt right up, but the diff cover is line bored to match a specific case so they might not line up properly without machining. You’ll also need gaskets etc

While you have the diff cover off I’d swap the bronze bushes for the shift rod as these wear out and cause slop

Have a look on youtube : Med race technologies - Project 63 part 13 . 4spd synchro box and 850 diff housing (they do install a s/c dogbox, but same principle.)

Yes that’s the shift rod. If you need to get all the parts you need:
Diff cover
Shift rod
Anti rattle plunger, spring, cap
Bottom plate, set screws and spring washers
Gearstick, retaining plate and set screws

And I’d suggest replacing the bushes in the diff cover