Gearbox interchangability

[Mark Gardner <mrgrdnr@ix.netcom.com>]

Hi all - What is the commonality between 1500 O/D and non-O/D boxes. 
When I was looking at cars, I had more than one guy tell me it was easy
to "just bolt an overdrive on the back end of a non-overdrive unit". 
True?  If not, just what parts are common?  Gears?  synchros? shafts?
castings?

Thanks in advance

Mark Gardner