My '84 has the same problem except without the knocking. Cold it has about 50 psi and goes down to about 10 or 15 psi when warm. It never varies with rpm (which is scary because i've revved it pretty high). I switched to 20W50 and the oil pressure is about 5 psi higher than when i had 10W30 in there plus it quiets the valvetrain down. Try putting a thick oil in there for now. Chances are if you can hear a knock, the low oil pressure from the main bearing(s) has caused there to be very little pressure to the rod bearings and you may have some munched up rod bearings as well as main bearings. For high performance applications i heard it's best just to get a new crankshaft with standard size bearings because as you go more undersize with the crank, the more likely you are to spin a bearing with it taking up more clearance where the crank used to be. Also, if you dig into your bottom end, clean off your connecting rods and see if any are discolored and replace any that have become dark or even black near the rod bearing end (if that's where the knock is from). I Hate to hear your misfortune.
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previous name: 84stangLX
'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT