Shes alive an shes stayin alive!!

I had a similar problem on my 78 Trailduster 400 BB. Ran fine for months after putting in the 400 then it started back firing when excelerating. It ended up where I couldn't keep running. after replacing the cam & repeatedly messing with the timing (i was getting spark too) it ended up being too wide of a gap at the pick-up in the distributor. After setting it correctly I was slinging mud without a problem. I would recommend switching to the electronic ignition too. once it's set right you don't have to mess with the points anymore.