On the second picture from below(1905 model) you can use the backside of the front receiver ring as a guideline for the amount of opening up the action on the front side.

I´ve added a picture from below showing an original commercial Oberndorf action that was made for the 7x64 Brennecke and the US 30/06.
The magazine boxes for these actions measured 88mm lengthwise opposed to the "normal" 84mm of the standard actions.

I believe that is the maximum without reducing safety.