383 Cruising Issue

The math says; 4000~94 mph with 3.23s and 27s. That sound right?

My guesses:Overheated sparkplugs due to;
A)wrong heatrange ...... Move colder 2 heatranges and/or
B) lean running,..............Fatten up the main jets 2 sizes and make sure the fuel level is and remains correct,and/or
C) insufficient timing........Make sure the Vcan is functioning,and the total timing is near 55*@4000@cruise.
D) engine running hot.
then roadtest.