Hello Team Chevelle, and thanks for your interest in my question.
I've been waiting for some time for the new Autogear Muncie-style 5-speed OD trans to come out. Besides mileage and engine wear, the biggest annoyance at 70-80 mph on the interstate is vibration / noise. I've balanced the driveshaft, etc.. but the rear-view mirror is a blur above 60mph. An OD trans will slow rpm's 23 to 33%, but the Out-put shaft/yoke/driveshaft/pinion are still going to be screaming at 3500-4000rpm with my 373's. I'm basically wondering if I will still have alot of vibration and should maybe consider 308's instead? I just like my 1-2-3-4 gears around town with the 373's and would like to keep them.
Any input would be appreciated.

The overdrive will make life easier on your engine, but you need to get to the root of your vibration. Pinion angle is a primary suspect, many times caused by a too thick transmission mount or worn control arm bushings.
Does your vibration improve when clutch is dis-engaged or when coasting in neutral? Could be anything from off center pressure plate to a bad water pump.

Hello Team Chevelle, and thanks for your interest in my question.
I've been waiting for some time for the new Autogear Muncie-style 5-speed OD trans to come out. Besides mileage and engine wear, the biggest annoyance at 70-80 mph on the interstate is vibration / noise. I've balanced the driveshaft, etc.. but the rear-view mirror is a blur above 60mph. An OD trans will slow rpm's 23 to 33%, but the Out-put shaft/yoke/driveshaft/pinion are still going to be screaming at 3500-4000rpm with my 373's. I'm basically wondering if I will still have alot of vibration and should maybe consider 308's instead? I just like my 1-2-3-4 gears around town with the 373's and would like to keep them.
Any input would be appreciated.

Do you also wait for the Tooth Fairy and Santa Claus??? There are other OD options out there right now.

OD will cut down on RPM and save the engine some work, it will reduce fuel consumption also. 3.73 gears are not that deep. somewhat deep but people have run lower gears. I think you have a fundamental issue somewhere in the drive train as outlined above. The rear view mirror being a "blur" indicates to me that the car has something seriously out of balance. Unfortunately those balance problems can be hard to find sometimes.

You could wait for the Muncie and if it ever shows up install it only to find there is stil some vibration. I think you need to seriously look at the driveline in your car,

Our Chevelle has 4.10 gears. Even cruising at 3500 there is no noticeable vibration. Off centre or worn out universal joints can cause a lot of grief.

I know about the other OD options, just always wanted that Rock-Crusher sound / shift, and don't want to hack up the floor. These old cars will never cruise as smoothly at 75mph as the newer ones, just wanted to know if merely slowing engine rpm will actually tone things down when the driveline is still spinning so fast.
Had the same loud ride and vibration with a small block prior to going Rat 10 years ago. Balanced 396/402, (not clear on the internal vs externally balanced stuff) balanced driveshaft, new pinion yoke, bearings gears, and near-factory ride-height pinion-angle.
I have the HP pulleys on the motor, so the water pump, etc are spinning slower.
I'll try the coasting thing when I get her out this spring, never thought of that...

Welcome to TC Jeff! :waving: [BTW, love the alias! I think that is from the episode where Homer is downhill skiing... love the mogul scene, lol).

What the guys are saying about tracking down the vibration I tend to agree with. The problem is there are so many causes. It sounda like you have addresses the usual items like drive shaft. Consider having rims checked for trueness too a slight bend or warp would be hard to see by eye.

I don't think an OD will help vibrations that are not specific to engine/clutch/trans input RPM

Thanks Ron,
And yes, you would have to be a life-long Simpson's fan to get the alias...
My kids are in their 20's now, and we still can't forget how hard we laughed the night that episode aired...
http://www.youtube.com/watch?v=CL4iVQyp9L4

I have a 68 Camaro with a small block 350 and 700R4, solely for th reason of keeping my 373s around town. The 700R4 allows me to only run 2500 rpm at 70-73mph. I also have the same vibration issue and have been through a drive shaft balance, new u-joints, checked the rims, etc. I never was able to solve it (after throwing $1K chasing it) and have almost chalked it up to harmonics. Let me know if you find the fix!