Horsepower is largely based on your top end.....
I'd grab a good set of CNC ported Edelbrock heads from Keith Craft and use a good roller cam to match it.
I'd also ditch the I-beam rods and go with some new H-beam rods for strength.
You can also gain some power and torque from some extra cubes. You can grab a 4.125 or 4.25 stroker crank from Scat.
I'm hitting right at $8500 right now in my own build.
I've accumulated most of the parts:
391 truck block with webbed mains
Scat 3.98 crank
Scat H-beam rods
Keith Craft Stage 1 Edelbrock heads
Comp custom grind solid roller cam
Crane roller rocker setup
10.5:1 compression ratio
Victor FE intake manifold
Canton road race pan w/ windage tray
Fidanza aluminum flywheel
Fluidampr harmonic balancer
Edelbrock water pump
MSD Pro-billet dizzy
Pro-Gram billet cross-bolted main caps
Demon Race Demon 750 carb
ARP Studs and fasteners
Clevite bearings
Felpro gaskets
Melling HV pump, ARP
oil pump drive
Stock front cover and pulleys
I think I'm gonna come in right around $9k on this build. I've found some really good deals on Ebay since I'm not in a hurry.
Desktop Dyno is showing 559 hp with this setup.