Converting a front U-joint
The ball-and-trunion joint ("Detroit style") was used through 1965 on A-bodies. It combines an angular flexibility with a longitudinal flexibility, in a single compact package. Starting in 1966, the much more familiar cross-and-roller ("Spicer type") joint was employed, along with a slip joint in the back of the transmission tail housing to allow for fore-aft motion of the driveshaft. (The yoke slides in and out of the tranny on splines, and this motion is needed because the rear axle tends to "wind up" when power is applied.)
The conversion seems easy, right? Just find a Spicer yoke to bolt on to the flat flange of the old tailshaft... But wait a minute, you need a slip joint somewhere, unless you're planning on updating the transmission at the same time, in which case this entire article is moot. Besides, if your car is an automatic, you'll need to keep the old-style (cable shift) tranny if you want to retain your pushbutton shifter, unless you want to do a whole lot of re-engineering. For what it's worth, the main advantage to updating the tranny would be an easier time finding speedometer cables, and a vastly superior availability of speedometer cable gears. (In which case, save your old ones... the purist restorers will want them!)
Note that Charlie runs with a driveshaft safety loop, since he doesn't want to do any pole-vaulting accidently!
Your local driveshaft shop should be able to build and balance a new unit for you, by welding together a new yoke, slip joint, and tube. The part numbers are shown here, along with photographs of the finished result.
thanks to Erik Ievins, for story and photos.