new engine problem

Just put together a 408 and got it broke in. Problem is when I went to set timing it will not run with less than 40 degrees of initial timing with vacuum advance unhooked. If you try to turn it down it will just die or wants to backfire through carb. Cam was degreed when it was installed,tdc mark on balancer is correct,tried another distributer and also double checked to make sure dist drive shaft is installed correctly. Any ideas?