67-72chevytrucks.com – a great information forum for 60-66 C10s also.
LT1Swap.com – the best in-depth online resource for those LS swaps
GMC Paul’s – Th info you need for OEM bed wood.
Jim Carter Truck Parts – an often overlooked resource for classic GM truck parts
Paintref.com – Paint codes for most years, makes and models. OEM codes and cross-references.
Various Facebook Groups. There are many specific groups available.