Main bearing rebuild

[Geegc@aol.com]

The main bearings on my 68 2000 went bad, and Les suggested that I rebuild 
the entire engine rather than just redoing the bearings.  That is a much 
bigger job, and I am seeking the advice of the list.  What do you think could 
cause the main bearings to go bad?  Do you think an entire rebuild is needed 
(based on totally inadaquate information). 

Gary Cohen
the red one 
the black one (in repair)