Yup! You don't want to put an engine together and find that the rearview mirror shivers in certain rpm ranges. A slight imbalance at 4500rpm can amount to hundreds of pound of force, beating your bearings, ripping up your motor mounts, cracking various things. An inline six can be made to run very smooth, if you're willing to spend the money and//or do the work. Fortunately, Ford Big Six crankshafts have been found to generally be rather well-balanced from the factory, so you could save money there. But you definately want to get your rods balanced end-for-end, and the pistons, and most definately the flywheel, if you have it reground. I went the cheap route (no balancing) a few times when I was young, but the third one shivered the mirror, and fasteners wanted to back out. Now I get complete balancing jobs done (incl. Big Six cranks); expensive, but worth it to me.