Hey, MARK!!! yes, of course it has to be balanced. If not then all that work and it shakes and can't rev. This is all on your 71 A65 FS motor that you sold me. I know you had that Trumpet out by now, hold it down to 90. On the plug affecting balance, nothing a little flywheel cut wouldn't correct, Rather use the oem style with the right size thread. I am guessing that some of the plug removal problems are due to mismatched thread.
Drive safe!
Bill