I have a 67 convertible 400 that recently had the engine pulled and underwent a complete engine compartment restoration. I was provided with several sets of numbers.
Front of block 29P283725 0635918XH
Rear of block 9790071 C219
Head cast 670
The VIN 223677U106739
I have been told that the engine was rebuilt using aftermarket releifed pistons. Aftermarket valve springs and screw in studs are installed. The mechanic estimated the compression at 9 to 1. Factory specs on the 670 should be 10.75 to 1. I'm scheduled to have the car Dynoed on 1/18. If the dyno results show less than expected numbers (330 HP) what steps should I take to get the engine back to it's potential?

Stock height pistons without reliefs should do it. They probably lowered the compression to burn pump gas. It should still have good power at 9-1.

That motor should be a 350 horse Grand Prix motor. 10.5 with an 066 grind.

Many variables, cam choice, carb, intake and final compression.

If you want to run a blend of 93 and 104, 10.75 is fine. If it is at 9.1, leave it.

You can reasonably get 450 HP out of these motors with 670 heads.

But if 330 is your goal, as long as the heads have a fresh rebuild, carb and distributor are dialed in and the proper cam is installed, I see 300-330 HP at the crank with 9-1.

Stock cam- Thinking of installing a mild Crower
Carb- Rebuilt Cliff Ruggles Quadrajet
Intake- Edelbrock, but I have RA III I was perhaps going to install.
If I can get 450 with 93 octane and a bottle of octane booster, I could certainly live with that.

Dyno came back at 255 HP and 353 torque (at rear wheels). With the TH400 tranny figured in I was told that would equate to around 320-330 flywheel HP. Does that sound right?

Sounds like it's tuned quite nicely. IMO that engine is just about perfect for a cruiser.

What are your thoughts on using different cams? This car is extremely well restored and probably will only get a couple of runs on the 1/8th mile track, but aren't we all looking for just a little bit extra?

In my opinion, you aren't going to get much from just switching cams. If it were my engine, I would leave it like it is or add some aluminum heads with a little higher compression. The third option would be to leave it as it is and add a Nitrous kit for the track.