I had a 235 in my 52 with the mallory electronic ignition.. the thing went on a blink and almost drove me crazy because it would not do it all at once but got worse...........it would just die, spit and sputter and then pick back up.........
on the hiway at 65 it would stop running and sometime pick back up after a back fire... I replaced the electronic module in it and never had anymore problem.........I doubt that you have one but a lot of back fire can be caused by electrical rather then fuel..
I had the same problem with my '66. It would do the exact same thing as Denny's.