I wouldn't drill and tap it, I would have a female o-ring boss welded in. You would need to rear mount it from the backside if possible... then weld it on the front side. You would obviously need to remove the baffle to get in there. Then you can use a 90 o-ring fitting. XRP has a crap ton of o-ring fittings, even low profile as seen below:
The nice thing about the o-ring fittings is that you do not need any tape and the seat is adjustable. So as long as there is clearance to spin the fitting around a couple times to grab thread, then you can tighten the nut down to seat the o-ring.