You might have two totally different problems. How do you know that your oil pressure's stuck at 85psi? I have yet to hear or or see that occur. The rear seal doesn't blow from high oil pressure, it blows from high crankcase pressure such as a stuck flame trap and/or clogged flame trap/pcv hoses.
Any Volvo part for a 740 (and/or even much older Volvos like 60's cars) can still be ordered from a dealer IF they want to. Not only do I work for a dealer (with 18 yrs of Volvo dealer experience), I own 2 mid-eighties 740s, one turbo and one NA. If someone told you that they can't get you a part, it's because that person is either in-experienced and/or doesn't want to bother with doing his job correctly as I know otherwise.
If you're guessing about oil pressure, put a manual guage on it or have it checked as I'd find that hard to believe. You might have 85 psi with a cold engine at higher rpms but as it warms up (takes about 20 minutes of driving), the pressure lowers and especially at idle. I've had cars with as much as 100psi cold and not a drop of oil ever leaked from seals.