IIRC it was with the body style change in '67. GMC used the 305 V-6 til then, and I think they also had a 351 V-6 even into the late '60's but won't swear to it.
And by the mid 70's Chev made everything from one ton and down for both badges, and GMC made everything from there up for both makes. I worked in a Chev dealer ship and we always got trucks in with GMC emblems on the Chevs. Sometimes it was just one side sometimes both on the fenders. Grills and tailgates were usually correct.
That body change brought the usual C/K (K for the 4x4's) 10/20/30 series for Chev and the 15/25/35 for GMC.