What I meant is, did the mechanic retrieve the psi value from a sensor scan or physical pressure gage mounted on the fuel output line? If it was from a scan, does the mechanic's scan produce the correct value from a car without the problem? Just want to make sure the
fuel rail sensor giving you the low 400-450 psi value is functional.
I'd also look at the discharge pressure from the fuel pump in the fuel tank to make sure it is within spec. Also, try to see if the mechanic can monitor and record the fuel rail pressure when you first start it up and then until it stalls, both for the psi value and to see if it fluctuates or is steady for a given RPM.
If the 400-450 psi fuel rail values are constant, and for both sides, I would look to an upstream problem (like maybe air in the HPFP suction) as it is unlikely both HPFPs suffered a mechanical problem at the same time.
Since you are dealing with very high pressures, I would take extra time and do a little more research on materials and techniques if you decide to DIY.
Please keep us posted with your progress as this thread will probably help a future 760 owner.