Oh Jim,
The method depends on how bad off the case opening is deformed. I have a slightly eccentric brass plug-like fixture that I slip in and rotate to straighten out the bulk. I then use a very sharp, triangular scraper to relieve and interference. Since these parts are closely fitted, the T mounts don't have to create much distortion to make fitting a problem. A little clean up work and all is well. Flatheads like Indians usually consider 11.0:1 an extreme C/R
Fred,
I too used to use self tappers in the plastic mounts, like Dave on smaller engines mostly. Gradually moved to using through bolts with "tapping plates" when using plastic T mounts.
Some may think "overkill" but it works very well and I don't mind a bit of extra effort. Crashed a few planes over the years but never lost a lug with the aluminum mounts and the girdle channel.
Much like some of my buddies in the old days would paint their bikes with rattle cans or brushes.
Indian Wally dipped his tanks in Rust Oleum
It is what we do.