Rebuilt 318 install problem

Mark your balancer from tdc line out to 60 degrees or buy a timing tape. I am suspicious of cross firing or plug wires swapped. I seen you mentioned Mopar coil? is it a stock type? I would try an e core coil or a high performance coil. Is the mechanical advance causing it to over advance when you are revving it up? Here is a link to the \6 site with in depth hei installation, it may be worth checking out if you've not seen it before. HEI Electronic Ignition Retrofit How-To - Slant Six Forum