54 GMC Man - Since the truck will immediately refire when gas is poured into the carburetor, you probably have a fuel delivery system problem.
Items to check:
(A) clogged fuel filter
(B) clogged (or missing) fuel tank vent
(C) vapor lock (I doubt this one if occuring after only 5 minutes)
(D) defective fuel pump
(E) partially clogged tank fuel pickup
11275 and 11280 are both universal replacement carburetors. Check this link for original carb:
http://www.thecarburetorshop.com/TRGMC.htm Jon.
