New 3.5" Aluminum Denny's Driveshaft

The new aluminum driveshaft from Denny's arrived in the mail today.  It essentially arrived one month after I ordered the driveshaft including the holidays and shipping time.  The driveshaft is impressive as it appears well welded, made of quality components, and is quite shinny.

The first thing I did after opening the package was check the weight of the new driveshaft.  I found the aluminum driveshaft is exactly 12 lbs.  The original driveshaft appears to be just under 3" in diameter and weights 16.2 lbs.  The difference is not exceptional but I suppose it is essentially a 25% reduction on rotating weight which no doubt makes a difference.

The new drive shaft appears to be about 3/4" longer from center to center of the u-joint openings (51 3/4").  I expected it to be between 3/4" and 1" longer so this is about what I expected.

While I can measure it all day, they know what they are doing so there is not a great deal of point in it all.  The main thing I wanted to have answered is what they use at the exposed yoke length at the transmission as it is the subject of some internet debate.  In this case, they have exactly an inch exposed including the space taken up by the extended seal coming off the back of the transmission.

My only concern after the installation is that there is only 3/4" clearance between the driveshaft and the exhaust cross over.  The exhaust cross over is 12" behind the center point of the yoke u-joint.  With the driveshaft overall length of 51", that means the rear of the driveshaft can move much further.

The rear axle can actually move farther as it is roughly 10" farther back from the center point of the rear u-joint.  This means the contact point is 12" back from the front u-joint but the rear axle is roughly 62" back from that location.  I'll need to polish up my math skills to see how much axle movement can occur before the driveshaft makes contact with the exhaust.

My issue is more when the back of the car raises creating more distance between the the body of the car and the rear axle.  I decided instead of trying to do the math, I would just jack up the rear of the car and take measurements of the gap.  After a little work, I found I can raise the rear of the car 4" and have just enough for a 1/8" shim between the exhaust and driveshaft.  At that point, the tires are not off the ground but they are close.

I think I am going to need to clearance the exhaust just a little.  I think that I can clearance 1/2" and gain plenty of space to get the rear of the vehicle off the ground without making contact.  I think I'm just fine for general driving.  It is just adding clearance for the extreme chance that I hit some incredible bump.

01/14/20 Update:
There was a fair amount of transmission fluid that came out the tail shaft when I removed the original driveshaft to take measurements and then again when I installed the new driveshaft.  Tonight, I drained all the transmission fluid and replaced it.  I used Valvoline Dexron/Mercon in the gallon size (#773636) although just under 3 quarts was all that was needed.

The old fluid looked a little dark as if there were something black mixed in it but not thoroughly.  I'm not sure what that would be exactly.  There was very little on the magnet drain plug.  I checked my records but I don't think I've done anything but add a little transmission fluid since I rebuilt it a while back.  It has not had many miles on it since but it is probably time to do it mileage wise due to the rebuild.