Is there anything you would change right away in this combo to get it more reliable, more HP, better fuel efficiency etc?

The engine is as follows:

Block: Merlin II 509ci
Crank: Callies
Rods: Manley H-Beam
Pistons: JE blower pistons (757F)
Comp: 8-8.5 according to engine builder
Cam: UltraDyne mechanical flat tappet (J3854)
272º/282º Seat duration
238º/248º @ .050"
146º/155º @ .200"
.550"/570" lift valve
.3233"/.3353" lift lobe
114 LSA
15 16 (lobe master #)

Heads: GM 188 Cast iron 118cc open chamber, 325cc intake volume
Head gaskets: FelPro Wire-Loc
Valves: 1.88/2.25
Rockers: Jesel Comp Series 1.7
Intake: Blower intake
Blower: BDS 8-71
Boost: ~5psi
Carbs: 2x Holley 9022 800cfm
Ignition: Mercruiser Thunderbolt 4
Exhaust: Mercruiser/GIL Hi Performance

According to engine builder (MDG Performance) it dynoed at 720-740HP@5600-5700rpm on 91 octane.

Will be switching to and adding:
Exhaust: Stainless Marine Gen III with 5" dry tails
Intercooler: Superchiller
Boost: 8psi
Cam: Hyd roller by RMBuilder
Bigger thermostatic oil cooler

Noone has any ideas?

Go with a slightly larger cam. I run a 598/610 crane 731 roller in my 509's and at 8 psi I am at 765 hp and over 825 torque

What color is it ? You may have the perfect motor!

It's Pantera green...

What size blower do you use?
Intercooler or not?
At what rpm's?

Thanks

Get yourself an intercooler and a Crane marine ignition.When you say Mercruiser ignition,what do you mean?

Yes, an intercooler is on my shopping list, as well as a Stainless Marine Gen III exhaust (which I just bought from Miamiboys here on OSO...). Will I really se any difference with another ignition?!

I mean a Merc Thunderbolt 4.

Of course an ignition will help.If you are using a Thunderbolt style your timing is set for a carbed engine.Can you imagine having a multiple spark ignition that you can set the same timing curve of the 600sc Merc, 800sc,525sc,900 sc and a heap of others with the turn of a switch.It also has a boost retard function and rev limiter built in.It comes in one box and is plug in to your existing harness.But best of all shipping doesn't cost you a fortune like it does me.Summit racing have them.They are about $291.Almost free.

Dave has been doing this for many years, I would not change anything, leave it set up as he gave it to you. He spent the time tuning it on a Dyno, so if you change it, you have lost that. I know he sets up his engines on the conservitive side but they live.