Yep, if the C body axles are tapered you can't cut and re-spline.
I ran into this on mine.
Here's a link...scroll down to the axle sketches
http://www.bigblockdart.com/techpages/875axle.shtml
Here's one on Ebay Eddie
http://www.ebay.com/itm/Vintage-original-60s-70s-Chrysler-Dodge-mopar-Plymouth-dash-compass-auto-gauge-/390717717671?pt=Vintage_Car_Truck_Parts_Accessories&hash=item5af8973ca7&vxp=mtr
I think you can still buy them new. That's what I did.
Check your local auto glass store. Mine was $600.00 plus $100.00
for shipping. That included the installation.
A new glass would be the way to go in my opinion.
My "glass man" ordered a new one for my 'Cuda
Cost me $600.00 plus $100.00 for shipping and that was the
installed price.
http://excelglasscenter.com/index.htm
Not factory but work great. Price of hose is per foot.
http://www.nostalgicairparts.com/air-conditioning/2-inch-to-2-5-inch-duct-hose-adapter-66.php
Here's a great thread
http://www.forabodiesonly.com/mopar/showpost.php?p=648335&postcount=5
You can spin the "nub" on the speedo where the cable wire goes
in with your fingers. Can't remember which direction....try both.
The needle should move or bounce if the unit is not broken.