Vibration at 2500 rpm

[Ethan0]

Looking for some help at figuring out a vibration/shake at around 2500 rpm. There is a small shake at idle but it smooths once I start giving it gas. Then at about 2500 rpm it feels like the whole truck starts shaking/vibrating pretty badly. It does it in neutral with the clutch in or out, while driving, and with the truck in gear and on jack stands. Always at about 2500 rpm. It seems like something with the engine is the problem. Could the balancer be the problem?
It’s a 1976 k10, 350sbc, sm465, and np205, all stock.
Any advice or things to check helps, thanks.

 

[fast 99]

clutch cover gone off center, fan clutch if equipped. 350's are internally balanced so doubt balancer is the issue. If it vibrates and stationary without being in gear that eliminates driveline.

 

[Scott91370]

Motor mount? I know they are usually felt more at idle but worth look at.