My engine builder just finished a set of engines for my Formula. I needed them to be "island gas" friendly but I couldn't get a dyno time scheduled prior to installs so I'm looking for estimated hp. Here's the build:
454 Mark IV, new, high nickel saved for this project
Merlin 320 heads, minor blending and porting
Merlin Intake
Holley 800 (9022)
Comp XM270H cam
Stainless Marine with HP500 risers
Roller 1.7 rockers
2399 forged pistons, told estimated compression would be 9 to 1
Forged crank, 7/16 rods etc...
Told to set up for 5,500 rpm

It should be a very dependable and sturdy set up but would be guessing at actual horsepower unless I can find someone to dyno from the drives

You've pretty much got a 420 clone there. That cam will give it a little choppier idle. Running on a dyno could show anywhere from 420 to 470 HP depending on accuracy and accessories and exhaust.

I'm guessing 450 tops.

I'll say 440-450 too, if tuned well.
I'm typically a single plane guy for BBC's but with that cam and heads I'd sway Air Gap RPM. Not as much of a biggie with twin engines as singles.. Just throwing my worthless (free) opinion out there.

If they are still around, call Land and Sea to see whom in your area may have one of their propshaft dynos.

I'm not a cam guy but according to builder the cam is several steps different than the 420/425 factory cam. He also says we can easily change to the bigger one if I'm not satisfied. I didn't want to go with the retrofit roller lifters simply because I had major issues with some
on a prior build. Cost me a new engine. They were originally going to be supercharged, hence the head size, but decided on going to simpler, less overall compression route. Any recommendations from experience??