402 High oil pressure

Ok here's the deal:
Just got my 402 back from the speed shop after rebuilding. First time I fired the engine only showed 20 psig on the electric oil pressure gauge. Shut it off and hooked up a 80 psi mechanical gauge. Fired it back up and the mechanical gauge was buried at 80 psi. Went and got a new 100 psi sending unit for the electric gauge, installed it and fired the engine again. Now the 100 psi gauge is buried at 100 psi. I realize everything is new and tight, but I think this is way too much oil pressure! 7 qt. Milodon oil pan, Melling high volume oil pump (the speed shop checked the valve in the pump before installing and it opened at 70 psi), gauge is plumbed in right above the oil filter, Castrol Syntec 10W 30 oil. Fram PH30 oil filter. Can't think of anything else that might help diagnose this problem. Any help or advice would be appreciated since I'm about to pull out what little hair I have left.

I'm assuming that this is a BIG BLOCK CHEV 402????????
Welcome aboard!!!!!!!!!!!!!!!!!

Do you have oil galley restrictor plugs???

Have you checked for oil flow far end of galley push rod/rocker arms????

Its not unusual for high volume pumps to put out excessive press as they usually are hi pressure also--but even if not, the oil relief hole can only bypass so much oil---and relief valve may be in pump backwards, main bearing upside down, etc

I ain't no engine builder, but had many built.
Been told-- DON"T BREAK IN WITH SYNTHETIC OIL
and get rid of the Fram filter (PURE JUNK)
Good luck

Many approaches, but what I did when faced with similar problem, I hooked up a mechanical gauge off(old Stewart- Warner I had) an engine block oil access port. I left it in the engine bay temporarily where I could keep an eye on it and not have to go thought bother of running lines through the firewall. I used a remote starter switch to fire it up and discovered my electric dash board gauge was not working properly.
A word of caution here, I knew I had oil pressure but was concerned because the original guage read too low even when cold and with 20-50 oil..the opposite of your problem but an approach to consider after you insure the oil filter and by-pass are up to snuff!

Yes big block. No, don't have restrictor plugs. Haven't taken the engine apart yet. Thanks for the help.

You need to find out how much pressure you actually have, so I would suggest an accurate test instrument type gauge (often available at parts store loan a tool) but it's quite possible you have a stuck oil pressure by-pass relief valve in the oil pump. Did you use a oil pre-lube shaft to supply pressure though the engine before firing it up? Also, I agree that engines should be broken in only with regular "dino" oil, but this is not what's causing your problem. Did you check the "high volume" pump clearances prior to installation? Still suspect the pressure relief valve.