Internally Balance Cast Crank?

When it rings it takes maybe 2 seconds to be quiet.
When it thunks, it takes no time at all.It's just dead.
It works best if the crank is standing on it's tail, and not leaning on anything, and no hands . I put the flex plate on backwards with one bolt holding it on;works great.
Curious, can you mix and match cast cranks,3.58" throw with forged flat top pistons in a 340 with H rods, and get away without balanceing it? Would it be coincided internal or external balance?