Well, the long saga with this vibration has finally been fixed. We ruled out the trans and torque converter by bolting up the bellhousing and starter and running the engine. The shake was still there. Then he dropped the pan to make sure all was well on the bottom end....which it was. Turns out someone had made a 50 oz. c4 flexplate by welding weight to the 28 oz one. Apparently the welt was not so good because upon further inspection, you could see weld marks where a weight had once been. Anyway, a new flexplate with the correct 50 oz. imballance cured the shake. There putting a new front pump seal on the transmission now and I should be back on the road and running right for the first time since the engine was installed, I can't wait. Also this expense has caused me to forego my AOD install. I got the AOD, but the case was cracked, so I'd have to get a new one anyway. I don't think I really need the OD anyhow since I won't really be doing any highway driving as I had originally anticipated. How much of a mileage hit do you think I'd take if I went ahead and installed the 3.55's I have for the AOD install, but kept my c4?