Forget opening the hole in the shield unless you have access to a milling machine. The hole is precision made so the bellhousing can be indexed to the crank.

I don't see why you couldn't turn down your bearing retainer as long as the bolt pattern is small enough so as not to be compromised.

If that won't work and you're stuck, I have some spare cases - both a cast iron and an aluminum one. Just swap all but the retainer into an older case.