The two things that stick out to me are:
- no mention of any head porting or oversized valves
- “Performer” manifold. If it’s the std Performer, not the Performer RPM there is some potential power left on the table there........ especially if the heads get reworked.
If the motor has stock heads with no porting and the std Performer, I can see where you’d have an under 400hp(at the crank) combo.

I find chassis dyno results can have an even bigger swing from facility to facility than engine dynos(and different engine dyno results can vary quite a bit).
So, without having something to compare to off that dyno...... your 304rwhp is just an arbitrary number to me.
Maybe the dyno service could give you some numbers from bone stock modern muscle to use as a comparison.

A pretty basic pump gas 440 with say...... 9.5cr, a mid-sized hyd cam, some mild head work to factory heads, headers, intake, carb, etc will be in the 425-450hp range at the crank.