I happened on a pr of NOS door sill plates for a C body the p/n # claim them to fit 74-78 is 74 a change over year or do these fit farther back to???

Your sill plates will not work with 1973-older C's

The 1965 to 1968 C's are known as "slabs".
The 69 to 73 C's are "Fusies" short for the "fusleage body style.
1974 was the last body style change that ran to the end in 1978.These are known as "Formals"
Post them for sale on the Dry dock or Moparfins dot com

THANKS A BUNCH, I WAS GOING TO HOLD ON TO THEM IF THE FIT 69-71 BECAUSE I HOPE TO OWN ONE SOMEDAY.