I had a rebuilt M76 in my VK temporarily with my Hot 202, The first time I didnt get out of the end of my street.
What is weak in them is the circlips that retain the gears on the main shaft. Also the width/thickness of the gears esp 1st & 2nd. My boxed destroyed 1st and second pulling the gears past the circlips destroying the gears & syncros.
Second time around I had a main shaft machined to take larger circlips. After about a week the gearbox exibited early signs of the same problem. I stripped it down to find markings against the circlips & excessing movement of 1st & second.
I put new circlips in and sold the box to someone putting it into a carbie 202 stocker.
What breaks these boxes is torque & load in 1st & second gears. Also loading 5th gear up isnt good for them either.
If its a good car I would recommend a T5 conversion, the V6 T5s & Ford 6cyl T5 boxes are getting pretty cheap now, and work very nicely using a cable driven clutch. The are also Celica 5 speeds & Supra 5 speeds to consider also, but my personal preference is the T5. If you dint mine a 1:1 top gear ratio you can't go past an Aussie 4 speed, 6cyl M20s go pretty cheap now and are very easy to rebuild with new bearings etc.
cheers
Scott