Why do they have horizontally mounted shocks on rear of
66 - 70 Cadillac Eldorado and Oldsmobile Toronado?

These shocks are there to increase the front to rear stability.
Just look at the overhang on the ass end of that car.
The shocks do basically the same job as the vertical shocks;
 (i.e. they control and eliminate motion).
 We offer standard replacements but highly recommend is 
 an upgrade to our extra heavy duty shocks
because it has a 1 3/8" internal piston providing
 approximately 50% more piston area to spread the load across.

 call 800-344-1966 or 970-513-8283 for more details.