Some easy checks:
Can you check what casting numbers are on the intake and heads? Should be c1ae-a heads.
Will the dyno be with the flat log manifolds or the 2" dyno headers? BOTH would be interesting but since he plans to keep the iron logs....
Is it (and will it be) a 4.05 bore or a 4.08 bore etc?
Harder check: I would degree the cam that's in there first, not much cost compared to dyno time, just pull 1 side's rockers & a degree wheel.
The 61-63 390/300 had 9.6 CR factory with a slight dish & valve cuts. About 9.3 CR with todays thicker gaskets. About 285hp on the Gonkulator.
The 64-65 390/300 had 10.0 CR with factory shims, no dish, just valve cuts.
I ran a quick Gonkulator on this one:
Stone stock 1961 390/300 Bird, log iron exhaust, big 186-186 .400 .400 cam:
Torq 391 at 2600
Powr 285 at 4300
This engine, with log iron, 9.3 CR, 600 Holley, and log iron:
Torq 377 at 2700
Powr 282 at 4300
And with the big 2" dyno headers (too big):
Torq 371 at 2700
Powr 288 at 4700
On the plus side, all the dogs and cats we've had tend to like THIS kind of engine. Mild, quiet, smooth.
They do NOT like solid cams, open or loud pipes, or high RPM's.