1968 340 timing

I am setting my 340 up,I have bnoticed that it doesnt "like" the 5 BTDC initial setting, at frist I thought my damper had slipped,but i verified TDC and the mark lines up. My engine seems to like 10 or 15 btdc. It has been converted to MOPAR electronic igintion with Orange box. It will run at 5 btdc,eith 16 inches of mercury on a vac guage, but it doesnt start smoothly,it long cranks. I am thinking it needs more advance.