So I finally got around to drilling and tapping the receiver to help the JB weld hold the sight on. I honestly don't think it would have come off but I wanted a guarantee. I have no drill press and used an Irwin drill and three flute tap for 4-40 in a hand held electric drill !!!going slow!!!. I used regular gun oil as a tap cutting fluid and it all went to plan. I used a tapered tap to thread most of it and when I got to my depth, I ground the taper off the tap and bottomed it out. There were a few moments when it got tight and I thought I'd blown it but it came loose and was fine. A $3.00 tap and $2.00 bit. I was apprehensive about trying it myself but it was easy so long as I went very slow... about an eighth of a turn before reversing and removing the tap to clean out the hole frequently with a shop vac. I'm feeling like I could drill and tap the same for a Williams or similar. The receiver is very think on that back strap. I buried a .375" long bolt and I did not burst through. Also remember the peep I made was actually JB'd in place and I drilled and tapped right through it in place... so it's also bedded now. Just thought I'd share.
Confidence to anyone who is having a hard time finding a rear peep for the CZ527. And wants to try this themselves.