As GM divisions became more corporate the more sharing took place. Any GMC with a gas inline 6 from the sixtys on will be the same as a Chev. The later style GM inlines started in 1963, but I am not sure if GMC had inlines at the time as they were very proud of their V6 motors.
The main point is it could very well be a 292. Just hope the seller knows the difference between a 292 or a low deck 250 or 230 motor.
