I'm installing a Quaife ATB in 7" Tripod Sierra diff, but there in some interference with the case on the left side. Is it normal? How to solve it? Is there a trick or do I have to grind to enlarge the opening?

Some fit some dont so as you say out with the grinder to the casing to get it in.

yep the interference is normal .
in order to get the ATB into the housing you have to file a small chamfer to the inside back of the left driveshaft hole in the casing.

After your confirmation, I will grind, hoping it will be necessary only a little bit.