After eliminating all that you have already checked;
First thing I would also verify is that at idle the harmonic balancer is not oscillating excessively; i.e. verify that it is still good. If you had a weak balancer allowing for excessive torsional oscillations, I doubt that the engine would have survived for 10 years - but still worth checking.
Secondly, If the angle between the transmission and the driveshaft and the drive shaft and the rear end; i.e. the driveline angles are not the same or nearly the same, one of the possible outcomes is a cyclical (or beat frequency) type of vibration. This means that the vibration varies in pitch or intensity, not only at different speeds, but also can be "felt" at times even when traveling at a steady speed. The vibration often "feels" or is "heard" as a "whir, whir, whir" type of noise.
A peculiarity of the standard 69 drive shaft (Discussed previously 2 years ago. Refer to the following link: http://www.camaros.org/forum/index.php?topic=914.0
) is that the standard drive shaft for '69 (not including BB with TH400) utilized an arrangement where the yokes from one end of the drive shaft to the opposite end are not "in phase" but offset by approximately 20 degrees. This sort of non-standard phasing makes proper drive line agles even that much more important.
You state that you've changed the driveshafts out twice. What did you install and how did you phase the yokes? Have you checked the angles between the tranny and the driveshaft and the driveshaft and the rear end? Are you using "taller" tires? Has the rear end been lifted at all? Taller tires, lifting the rear end, etc. can affect the pinion angle.