Flywheel for 408

Depends on how the machine shop balances your rotating assembly. If they internally balance it, and your flywheel is for an internally balanced 69 340 then yes, it will work.

I built my 408 from an early 70s block. However, if I were to do it again I would start with a roller cam 5.9 block (not sure when they were introduced) to take advantage of the many benefits of the roller cam. This will be cheaper than putting a roller cam in an older block..... ask me how I know, LOL.