M , back end noise (crownwheel pinion setup)

I measured the backlash on the crownwheel as 55 thou. When I assembled the gears in the new casing although the book says 12 thou. is the correct figure, I put it back with approx 35 thou backlash. I also cut the last 3 shims that I put back in both sides to enable easy acess to do quick adjustments if necessary.

Although the gearset in the old casing ran nice and quiet, they now sound like a fire siren, and scatter the livestock for miles around. I've moved some of the shims across to open up the backlash to no avail. I've been told that to set up the crownwheel and pinion as per the book would be a waist of time as they need to run in exactly their old position.

Anybody have the answer?

Ive also heard that this is a particularly tricky one, common to Farmalls. Someone must have done a few of these and know the tricks! Still wouldn't swap it for another colour machine!!!!