Re: 20moa Base and XLR Chassis
Any of the standard, non-extended, bases should work.
Ken Farrell's bases are top notch and a little taller than most bases (.430") This might give you just enough height to use your current rings. I ended up using a 0 MOA Farrell base with 1" Nightforce rings. My outer objective diameter is 2.36" and it puts me almost exactly 1.5mm (.06") above the rail.
If you haven't seen it already, Kyle has provided a lot of info on the website to calculate what ring height you will need. Obviously you can manipulate this by changing base height.