Okay - It looks like we are getting to the bottom of things here.
The distributor is most likely not the culprit in the retarding of the timing. What WILL cause the timing to retard is a worn / stretched timing chain. The rule of thumb on most street driven cars is 8 degrees of slop (which is +/- 4 degrees from zero)..
* With the ignition off
1) Remove distributor Cap
2) Put a socket / breaker bar on crankshaft / Hamonic Balancer bolt
3) Align timing mark with 2 or 4 degree spot on timing pointer
4) Rotate the crankshaft until you see the rotor move
5) Note the position of the timing mark to the timing pointer
6) Reverse rotation of the crankshaft while you are holding the rotor.
7) Note the position of the timing mark to the pointer.
The difference between the two is how much stretch or slack you have in the system.
This applies to gear-drives too. A customer brought me a gear drive to install in his SBC and it installed fine but had 6 degrees of slop in it from the factory.
Of course it was built by Hashimoto-Chang-Speed-Engineering in Taiwan - not Pete Jackson..
On your Rotor - make sure you have the correct year rotor for your distributor. They did make a number of electronically controlled HEI distributors that had no vacuum or mechanical advance and the rotors, as I recall, were different. Yes, the length of the screws (as you found out) is important as well.