86 gmc

i installed a target 350 in my 86 gmc in place of the 305 its a generic 350 well it vibrates now and was wondering if it is balanced differently would appreciate help

Im not familiar with target engines, did u do any of the work urself or is it a crate engine? Check the engine mounts they may need to be replaced also the carb might be makin it run rough, what are you running for a carb? Another would be, if u pulled the tranny at the same time? because then you may get driveline vibration from the driveshaft. Other than that should run almost the same as the 305