Erratic tacho can send you down the wrong track, espicaily when ther are about to die.
Sounds like a possible air stall, thinking 45s, big cam, high vacuum low flow, then butterfly's open!!
Check no leaks on inlet manifold or piping,(carbs usually spit alot)
Check if all accel pumps working in carbs.
If you are going to change cam timing, take note of the orignal setting so as you can go back to it.
Try setting it to equal lift on overlap as a starting point, and check piston clearence to valves each time you change the cam setting before starting engine by turning it over by hand for two revolutions, just in case.