Frequency of the noise...  Almost same speed as a muffler drone, but not noisy, just noticeable.  Best description is like a fan belt that is fluttering due to too long a distance between pulleys, but I know it is not the belts.

I have had at least four different harmonic balancers on it through the years.  Even if you match the revs to what they were when the engine was pushing the car, the vibration goes away when the clutch is pushed in.  I just visited a transmission shop with an experienced owner.  Next week we are going to put the car on his lift and run it with a tech in it so we can look under it while at speed. 
I guess I could start removing parts until the vibe went away.  At least I am fairly sure the car is not destroying itself while driving it.  I used to visualize an axle bearing about to come apart.

I'm sure you've triple checked this how is the drive shaft angle?

What RPM are you turning when it occurs?  Do you notice the same vibration when you drive at steady speeds at the same RPM in other gears?

6sally6 wrote:

Sounds you have covered almost every thing.
IF you drive it up to 80/85(in 5) the vib goes away,right?! Do that and push clutch in.....let it coast...When it goes past 73/vibration speed it vibs and then disappears means its not engine,right?!
6s6

It does not go away.
 

I am going thru the same issues with my 70 FB, a harmonic vibration comes in at 72 mph, I haven't gone to the extent that MS has ..... Yet ! Just installed a different T5 but haven't road tested it yet, but I was getting the vibration without the driveshaft connected and running thru the gears, I had a slip yoke with twisted splines tapped onto the output shaft so it was slinging oil out. The vibration was slightly there in fourth and really bad in fifth.

Incidentally I talked to a transmission shop that they guy told me 3.70 gears are notorious for setting up a vibration, that will be my next step pending the transmission is to change to 3.89 gears to see if is correct.

All the good ideas have been said and I know this is an obvious point, but in the long list of replaced items, have you tried a fluid damper?  Or have you used the same damper in all 3 replaced engines?

Sorry I missed the 4 replaced dampers letter in the thread.

Why can't you gear the car so it runs at a lower RPM range where no vibration happens?.
 With the engine you have in the car it surely has enough torque to utilise a lower numerical rear gear ratio.
 My 331 has nowhere near the power that you have but with the 3.25 gears it spins at 1950 RPM at 70 MPH. In top gear I can ease in the throttle so as not to down shift at that speed to pass  or maintain speed up steep grade roads.
 Do you actually use 1st or 2nd  with a 3.70 rear gears?

Rudi wrote:

Why can't you gear the car so it runs at a lower RPM range where no vibration happens?.
 With the engine you have in the car it surely has enough torque to utilise a lower numerical rear gear ratio.
 My 331 has nowhere near the power that you have but with the 3.25 gears it spins at 1950 RPM at 70 MPH. In top gear I can ease in the throttle so as not to down shift at that speed to pass  or maintain speed up steep grade roads.
 Do you actually use 1st or 2nd  with a 3.70 rear gears?

Rudi-Tudi........you KNOW the answer I'm sure.
Of course he uses 1st&2nd gears.......they "take away your breath" as you row full throttle through both of them!!!
THAT'S WHY its "gotta hefty gear" in it!
6s6
 

Triton wrote:

Here's my two cents, I had a terrible drone in my car that I thought was a bad vibration come to find out the exhaust was mounted too ridge and would transmit this through the car. I put in a modern style exhaust rubber and it's like a whole different car

 
Those are the exact same exhaust hangers I have in my car.  Three different exhaust systems. Same vibration at same speed.
Plus, have used several different gear ratios, 3.0, 3.25, 3.5, 3.7, 3.89 and had same vibration at same speed. Anything over 72 mph.

He's had the vibration with a C4 also. So, yes.

The damper I used came off a 2001 T5. My 87 T5 came with one like AL posted but I couldn't find it .I had a 2001 in the junk yard and it was a lot more compact.  Moved it back about a 1/2 inch and bolted it on .Cleared everything.