Basically the interchange is:
'73-'80
Fenders, hood, doors (trimmed for 73-75 full top), most interior, Grilles, radiator supports, inner fenders, etc..
'81 - '91
Radiator supports, Hoods, Fenders, grilles, doors (same as previous) inner fenders, interior.
Tops will interchange from '76-'91. '73-'75 are full top.
You can change '73 - '80 to the '81-'91 front clips with some work. I am doing an article on this site for the Grille upgrade to an '89-'91. You can also change an '81-'91 to the earlier grille.
What are your specific interchange questions. I am sure some of us can answer them.
<font color=red>Mudzer</font color=red>
1978/91 K5 Blazer