So I took another shot at it today. I removed the tubing and noticed it had flattened somewhat, which is no surprise as it was not designed to hold shape in compression. I decided I should look for something to insert into the the tube to give it more strength, and found an old power cord that was just the right diameter. Once I had that together I put a layer of RTV (Permatex Ultra Gray) on the outer and inner walls of the bushing, and then pushed the tubing into the slot. Then I filled in more RTV on the surface. Note that the slot is towards the rear, I just kept remembering it wrong.
Anyway, I put a heater under it and let it set for a few hours, then took it for a drive. Big mistake! I made it way too stiff/rigid, and you could hear and feel everything going on down there, from engine vibrations coming down the driveshaft to the clutch opening up, and the clutch vibration that is the source of this whole problem. It didn't really resonate the way it had, just could feel and hear it. That was unacceptable so I pulled it apart again (the RTV was not totally set yet).
I put it back with a fresh piece of tubing with nothing in the center. I think it's an acceptable compromise. On occasion I can hear the vibration between 40 and 45mph, but it's much more damped and does not shake the floor. Most of the time you don't hear it at all. Not sure what the random element is - perhaps the position the draveshaft stops in? Anyway, it's one heck of a lot better than it was.
I have no doubt that this bushing/isolator could be tuned so that it never does it, but I don't have that kind of time to spend on it. Time will tell if the rubber tubing crushes down and stops damping out the driveshaft.
The fuel line I used is about 0.560" OD. I tried some 0.625" stuff, but that was too wide to go in.