When you break in a newly built engine with a flat tappet hydraulic cam you have to run it at a fairly high, but changing rpm for about 30 minutes so that the cam breaks in properly . At that point the timing, mixture and other settings are only a best guess, and the engine may run hot because of the RPM, insufficient fan, or because mixture is off. The header company does not want you to screw up the new headers from running too lean and too hot during break in.
If all you did was pull the distributor, just put it back in as close as you can and restart the engine. Rotate the engine and line up the timing pointer with the 8-10 degree BTDC mark on the damper, using your finger over the #1 cylinder to verify it is at TDC. Then drop the distributor in so that it points at the #1 wire on the cap. Usually the rotor will point about toward the front corner of the driver's side valve cover, so you have enough space to rotate the distributor without the vacuum can advance hitting anything.