Is it normal for a 305 to start vibrating around 4500 RPM? And by vibrating, I mean before that, there's nothing, but around 4500 it starts to vibrate slightly, nothing major.

 

I had the same thing in my 350. At around the same RPM. When I pulled the engine, I found a broken motor mount which was part of the problem. I also found out later that my flexplate was shot. Could also be your problem, but I'd replace the mount first and see what that does.

EDIT: I see you run a manual tranny ... ok ... flywheel

 

I have poly mounts, could this just be transmitting the vibration more?

 

Perhaps, also check the tranny bolts. An old oldsmobile I had vibrated somewhat like that and found all but 2 bolts were loose.

Could be many things, from the clutch, flywheel, balancer, loose bolts, motor mounts, etc. Just have to narrow it down if it bothers you that bad. Mine wasn't very noticable. But there.

 

Doesn't really bother me, I just want to make sure that nothing is going wrong.

 

I suspect that your poly mounts might be the cause or helping the cause. I replaced my tranny mount with poly and it vibrates like mad now.