The main point of suggesting a cranking compression check, is that if the cranking compression is way off, it's likely that the cam timing is way off.
It's a simple, nothing-but-spark-plug-removal way to verify cam timing. Another method would be to check the rocker arm position on #1 at TDC-Exhaust (overlap) to see if the intake is open just a bit more than the exhaust. Problem with that is that hydraulic lifters may bleed-down making the check unreliable.
The simple fact is that with the info provided, it's fookin' near impossible to determine what (if anything) is wrong, because there's been NO TESTING DONE. A bunch of impressions, and some (vague) listing of parts used, and no real diagnosis. It's hopeless trying to list all the possibilities this could be. With regard to parts installed, what does "bigger" accelerator pump, "bigger" power valve, and "bigger" front jets mean? We know the timing chain has slack, but not where the cam was installed to begin with. We know that it's got an electric fuel pump and a pressure regulator, but we don't know what the fuel pressure is. "Roller rockers" that "might be Magnums" which are not roller rockers. (roller TIP rockers.) A "custom curved" distributor but not what initial timing, and the rest of the curve is. We don't know if it's got vacuum advance. The list goes on and on.
Now, a '67 Firebird is a relatively light car, with 3.73 gears, a deep first gear, a "high stall" converter, a "too small" carb, and a "too small" intake manifold. That thing should launch like it had JATO rockets. Maybe the power tapers off as the revs build, but stab the gas from a stop, and it should be nearly uncontrollable. Until the transmission shatters, anyway.
|