im chasing a similar demon in my 74 duster; 360 with 727 and 8.75. it vibes about 40-50, while in neutral or drive, the humming bee.. hmmm---hmmmm---hmmm.
so am i,just put in a 318/727 in my 73 duster. a matching set out of a 67 coronet. took 4 inches out of the driveshaft,professionally built and balanced,all new u joints. tires are all balanced. exhaust pipes are mounted with hangars. only vibrates moving at and above 55-60 mph.i havent pushed it past 80 to see if goes away.did not have a vibration before the 225 swap out. this is the original 727 to the 318 so there are no converter or balancer issues, and it is just exactly like you describe hmmm....hmmmmmm....hmmmmm... lol post up if you figure it out !