Q: I have a 1961 GMC 1/2 ton PU w/ a ARMY 305 V6 engine what fuel pump do I need on 1990 GMC 1500 Pickup

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The spark plugs are on top of the heads, which tells my freinds it is a military engine, they say it is a ARMY engine
(1) Answer
I'd say, go to an old-style (not a franchise or big name) auto parts store where hopefully they've got old paper catalogs and ask if they go back to 1961. If so and there's an old geezer there, they may be able to tell you what fuel pump it needs.
Another idea is to take out the old fuel pump and go to the parts store and match it up. It may be the same as a GMC or Chevy motor of the same period or later.