273 not pulling past 4000

Ignition? Points? Breakerless?

Adjustable rockers? Where are they set? Who adjusted them?

What cam? Reputable builder? Did they test/ replace the valve springs?

You run a compression test? Leakdown?

What you have for an exhaust system?

Is the cap / rotor new? How they look? Wet? dirty?

Checked the plug wires? AND THE COIL WIRE?

LOTS and LOTS of places for this to go "downhill."