I don't know if this will help you or not but when I built my 406 I didn't have the original balancer. So, knowing that the TDC mark was correct I advanced my timing until I got the highest vacuum I could attain (14") and backed it off one inch. Using a timing light with advance capabilities I determined the timing was @ 17 degrees BTDC. If I set the timing for anything less the engine sat there and loped and shook the car too much to allow comfortable city driving with my Broader C6 transmission. I then had a smooth idle at 650rpm. But, every time you tried to start the car it was quick to tell you the timing was advanced too far for easy cranking. So, to fix that I bought a Duraspark distributor setup that retards the timing 5 degrees when cranking the engine. It worked great. Jim