I feel like an idiot...

i had the same exact thing happen with my ol 73 D100. it was originally a 318 truck with a 4 speed. someone dumped a 360 in and used the teener flywheel and 78 vintage 318 balancer. It was obvious this truck was hashed together long before it hit my hands. good times were had after i figured out my hellacious vibration at 60 mph.