Check the points .I changed my points and the new points had a copper spring as well as a loose piece of spring that I thought was for a different application .the loose spring piece fit in with the copper and without it does not put enough pressure on the points and what was happening is when you accelerated it backfired because the points would not have enough pressure on the distributer shaft and would bobble around if that makes senice. Took me a long time to figure it out as I also was getting the engine running after a year of sitting.

1954 Canadian GMC 9300
"The Journey"
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