I think it entirely possible a slightly out of balance drum could create what you describe.
It is also possible that tires/wheels that were not lug centric balanced, or an out-of-round tire could cause the same thing.
Given your soon to be conversion to 4 wheel discs however, it is an issue that needs to be addressed after the conversionn is finished.
Charlie, CMSGT USAF Ret. 65 Hi-Po F/B, 67 GTA F/B (SB) Mustang Owners Club of Austin (MOCA) TX, Member MCA #27479. Now working a '69 M code mach 1.