As far as I know the DCS P-51D is limited to only 67" with WEP. I have seen it higher but that was only after getting shot up and breaking parts...
From the DCS web page.
"War Emergency Power (WEP) mode, nominally limited to 5 minutes of operation. WEP can be mechanically implemented in a number of ways. The first option is to artificially lower the pressure acting on the aneroid by opening an escape line, resulting in an opening of the throttle valve by the regulator so as to “maintain” pressure – while in fact boosting it beyond the value set by the throttle handle. This method was used on early Mustangs, which featured a special control handle in the cockpit to engage WEP. Another option is to design the throttle linkage assembly such that the relay piston is in the fully closed position when the throttle handle is set to full military power. The pilot would then push the throttle handle past this setting into the WEP position, further opening the throttle valve and the relay piston would be unable to act upon it to close it. And the final option is to design the linkage system such that the throttle handle position past full military power would produce manifold pressure up to 67 or even 75 in.Hg.
Given the limitations of most HOTAS controllers used by virtual pilots, DCS Mustang will model the first method. This allows us to avoid having to rely on throttle detents or limit their range of movement in the pre-WEP range. As such, we will have a dedicated input command to engage WEP as a simulation of a cockpit control handle."
https://www.digitalcombatsimulator.com/en/dev_journal/dcs_p_51d_mustang/