I don't have a definitive answer for you, I only know enough about dually rims to get me in trouble.
Here are a couple of links that may help... They're for aluminum wheels, but it should give you an idea of what the specs are and what works.
Alcoa wheel, this should be the correct specs for the dually rims for the 70's and 80's GM trucks.
http://www.alcoa.com/alcoawheels/north_america/en/product.asp?prod_id=1160&cat_id=757This is Southwest wheel, they list the same aluminum wheel for '76 to 2000. It seems that the lug nuts would be the difference, metric versus standard.
http://www.southwestwheel.com/store/c-1027-chevroletgmc-3500-dual-rear-wheel.aspx Here is another interesting link for GMC motorhome wheels that reference various years.
http://gmcmotorhome.info/wheels.html#STEELAccording the the motorhome link, '76-2000 C/K-30 (or 3500) dually, G-30 (dually vans) and P-30 (e.g., breadtrucks) should interchange.
Maybe someone has a Hollander book and can give you some more definitive interchange information.
Hopefully this should help some, keep us posted.