Still no resolution on the vibration. I was really hoping motor mounts would clear it up. They were dry rotting. So, again, it was good maintenance, but not the cause of the vibration. For the hell of it, I swapped the plugs out, and the old ones looked ok. Definitely a colder heat range than optimal on the street, but they weren't fouled, and again no change in vibration.
I noticed the timing is a bit jumpy when the spout is in. i.e. I'll set the gun to 40*, free revving, the timing comes on up, but a once every second or so, it pops to a different timing. Seems a bit erratic. However, with the spout out, it's totally smooth and stays at 10*. And, the vibration is still there, regardless. It sounds perfectly fine. High load, low rpm (maybe 1600-ish) WOT in 6th gear is perfectly smooth. Tried bypassing the MSD, too. I'm not totally satisfied with the timing jumping like that with the spout in, but that really doesn't seem to have anything to do with the vibration.
I test drove it and decided that if it was gonna let go, then so be it. Feels strong. Can powershift and turn the tires over. I can bounce the car off the limiter for 10+ seconds in 2nd gear. So, I really don't understand what's doing this. My best guess is maybe it is flywheel or clutch imbalance related since I installed them in January about 7k miles ago, but I just don't understand why it got worse recently.
I called my local machinist. I'm gonna bite the bullet start the 306 build. I have a spare OEM block with rotating assembly on the garage floor, some good top end parts, and I've been jonesing to get into it for a coupme years. The machinist thinks he can get to work on it in October.
That's fine by me. I'll keep trying to work through it, but as long as it doesn't get any worse, I'll take my chances to run Larry at foxtoberfest, this year. If it blows, so be it.
Edit:
I thought a pilot bearing might explain things, but after rethinking it, I'd expect to detect differences when in vs out of gear. Oh well... Will keep noodling.