Help! High pressure at the Dyno

So, they fired it up and it hit 100psi at 3k. Builder, not original builder and did not assemble the bottom end, said we can pull it off and start by looking at the pump. Or, since we have it here, put in thinner oil and see if it holds steady or climbs very little up to 5k. I'm not there at the moment and am trying to make a decision on the fly. Thoughts? Anything constructive is welcome. Thanks

Here is the setup:
408 stroker
LA 360 Bored 30 over.
Full roller.
Eagle 4" Crank.
Ross Pistons.
MAKE BORE STROKE ROD LGTH DOMECC DOME HT. GRAM WT. C.H. PART NO.
360 4.030 4.000 6.123 3.0 11.5 479 1.459 99789^
Comp Cams 20-740-9 Xtreme Energy™ Mechanical Roller.
Edelbrock Performer RPM's. Surfaced and stage III ported.
Edelbrock Victor 340 Intake Manifold. Port matched.
Holley 4150 850 carb.
MSD dist/coil/wires.
3/8 pushrods.
ARP head bolts/main studs.
Isky Valve Springs 9265.
Billet Timing Set.
Melling High-Volume Oil Pump M72HV.
Milodon UP18346 Pick Up
POWER BOND BALANCERS PB1004-SS.