Jerry - After searching through the few GMC Maintenance Manuals I have so far, I couldn't find the deck height spec listed in any of them. Some web surfing located this Inliners thread reply post that indicated an undecked 270 was measured at 11.3667". Mighty close to your 11 3/8" (11.375") SWAG.

Hope this may be of help.

~ Dan
1951 Chevy 3 window 3100
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"My Grandpa Carl's Truck and How it Became Mine"
1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver)
US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)