You're best guess please.

TT5.9mag said
Was it overdriven 6% on the 340 and made 9-10psi? If so it won’t make the same boost on a 408 with better heads. Likely 6-7 psi. With your cam, heads and 408 on 10 psi I’d guess 630-670 for a dyno number.

Outasite said
Jack,thanks for the reply.Was looking at my paperwork on my 340 today.I built that in 1995 and have had -0- issues with it.Not a ton of miles, but two tons of fun.Kept it to 6/7 psi and just never touched it.what kind of boost do you run,and do you have manifold air temp.I’m going 6% over so thinking I mite want to look at temp in the plenum.Oh, timing back then didn’t understand that boost don’t like timing.And was running as much as 35% total,but also a fair dose of 100LL most of the time.
Bob

I don't know exactly how much I am underdriven because it's been a while since I changed my pulleys. ( I have 5 of them) It is underdriven but close to 1 to 1 (maybe 41-40 pulleys) and I am making 9 lbs of boost. My "X" heads are ported as far as they can go with 2.02 valves. I had changed my top pulley to get 12 lbs once. That lasted about and hour before I broke my harmonic balancer. Sure was a fun hour though. lol Live and learn.

Jack