If it makes the pressure cold,I'm sure the bypass is moving lots of oil. A positive displacement pump will exploded an oil filter in like 1 second @ start up without a bypass. LOL I watched it happen to a bud who
insisted on putting an unknown heavier spring in his pump, lets just say that idea was all wet.
On the flip side of that, if the engine is using that much volume of oil inside that it won't reach or get close to the by pass point spinning high, hot, I'd be looking for more volume from pump and of course pan too, if clearances are in order.
Nothing seems out of order from the
info provided on the build. The lack of main clearances and and the rod bearing room all exactly the same,interesting, I don't recall seeing a end play #, or side clearance on rods.
Anyway, this does seem to be a clearance issue, as it is temp dependent. Whether it be bearings or lifters to bore or pump. You had mentioned rear main seal had to be replaced right before you got the car, so that cap was off.
If your going inside after talking with Butler, do your self a favor and just pull it out and have ALL clearances checked(yank crank and check with tools not wax!!!). And make sure rods are installed correctly on each journal, not 180 out. You will be more confident turning it 6k..... where you have you red line set on your tach.
Good luck.
Edit..man I type slow!!!
Your headers may be doing a number on that sending unit also.