What are the differences in the timing covers between the various years of
small blocks?

Answer (Aarom McDonald)

TYPES

First Type: 67 no timing tab cast into cover. The timing indications are
on a little plate that bolts to the water pump. Water pump
exits driver side.

Second type: 68 and 68- timing tab cast into cover, passenger side.
waterpump exits driver side.

Last type:  70 and up- timing tab cast into cover, driver side. water
pump exits passenger side.

Note: Of the very few interchange parts between the dreaded
poly 318 and our LA 273, 318, 360's is the timing cover, it's made of cast
iron.

INTERCHANGEABILITY

All covers and waterpumps will bolt up to each other, but only the
matching combo (of timing cover, water pump, balancer and radiator)will
ensure that you can:

        - see the timing marks (ie not blocked by the the lower radiator
hose/water pump outlet);

        - have the timing mark on the balancer line up with the timing
indicators cast into the timing cover (ie if mismatched components they will
be out of phase by 90 degrees).  This problem can be solved by remarking the
balancer, using MP timing tape, or getting a bolt on tab for it;

        -have the water pump exit on the same side as the lower radiator
inlet.

IMHO (Aaron's) if you're building everything fresh, go with all the 70
and up stuff (ie waterpump, timing cover, balancer and radiator) - it's
easier and cheaper.