Hi TrxR,
Old thread so I don't know if you already got all your questions answered? For your question above, I inserted two measurements in a photo attached of my Near Mfg Alphamount 34mm medium height, on a Near 20MOA ss rail, mounted on my Tikka T3X. In case it doesn't link, center-to-center for cross bolts = 3.55", and inside space between top ring caps = 2.45".
This is brand new received early this spring, and I only have only had the opportunity to shoot it in two Range sessions before my local Range had to covid-close. I have not switched it between rifles, or switched out scopes. Near Mfg claims their Alphamount has extreme repeatability when replaced on the same rail. Folks in posts above have mentioned that its working for them. The mounts are being used by some winning F-class shooters, so that tells you something.
The Alphamount is beautiful, fit and finish are superb. I really like it.
IMO the design for how exactly how the rail fits with the receiver is critical. Near's Tikka rail has the recoil lug milled into the rail. Near also provides different length screws for each of Tikka's receiver screw holes to maximize thread contact but not protrude inside the action. That attention to detail impressed me.