DWC's 1968 Camaro project

[F'in mine]

I could be wrong but this is my guess regarding the vibration. The flywheel you have is for a 1970-80 383/400 ci. in. stroker engine that's externally balanced, providing your internal crank is from the 400 engine with spacer bearings to acheive your stroker combo which I don't think you have. The 400's rods are smaller in center to center distance @ 5.565" & pistons have a 4.125" bore therefore the crank is balanced accordingly.
Your stroker could have aftermarket rods 5.7" or 6.0" with 4.000" bore pistons x 3.75" stoke as well.
Most stroker aftermarket 2 pce. rear seal cranks are internally balance but they do make an externally balanced one.

Here's an option before you dig into the internals. Get a flywheel & balancer that's internally balanced and see if it ails your vibration issue. I realize its quite a bit of work and you have to decide whether you want a billet flywheel and good balancer or the inexpensive stuff to try? I say get the cheap 150.00 stuff first to try it out. Down the road when you decide to balance the reciprocating assembly splurge then.