Much of the information above is good.
But for the umteenth time, I need to point out a couple of things that CONTINUALLY get mentioned incorrectly.
They say pictures are worth a thousand words, so here goes.
Tail shafts DO NOT, repeat, DO NOT have numbers on them!
This is a TAIL SHAFT.
This is a TAIL HOUSING (or more correctly, an extension housing), driver and passenger speedo versions.
THERE IS NO SUCH THING AS A SMALL BLOCK OR BIG BLOCK BELL HOUSING!!!!!
Bell housings are mated to an appropriate flywheel! There are/were TWO flywheel sizes: 153 teeth and 168 teeth.
GENERALLY speaking, big blocks got the larger 168 teeth flywheel. Thus, for the 168 teeth flywheel, the appropriate bell housing was either the 444(1966) or the 621(1967-later).
The 421(63 ONLY), 383, 403 housings were used with the smaller 153 teeth flywheels.
All Chevelles with a big block got the larger 168 teeth flywheel, thus, they also got the larger bell housing. But some cars, such as Vettes with a 396 or L88 got the small 153 teeth flywheel and 403 style housing.
Now, with that said, depending on which size flywheel is used, there is an appropriate mating starter required. Small flywheels use a starter with a straight across bolt pattern and big flywheels use a starter with the staggered bolt pattern. Also, if using a GM Delco style starter with a GM bell housing for the big flywheel (444 or 621 syle housing), then a starter with a CAST IRON
nose and staggered bolt pattern must be used. There is also a starter with an alum nose and staggered bolt pattern, but that will only fit in the bulge of an auto tranny. The alum nose, staggered bolt pattern is bigger than the cast iron nose and WILL NOT FIT in the starter bulge of a 444-621 bell housing.