1968 340 harmonic balancer and main pulley

You need a balancer for a 340. then. I would replace the balancer first. The pulley might be ok. You could start a wanted ad for one. I would look around for a new one. Many of the old used ones aren't in any better shape than the 45+ year old one you have. Here's only one place that has many choices. 1970 Dodge Challenger Parts | Balancer | Classic Industries