Great old step van! What's wrong with the bumper?
The GMC V6 is huge for a 6 cly and are known for lots and lots of torque. The 305 was the smaller displacement GMC V6, they made them as big as 479 but the usual size was a 351. Smooth running. They made one block and just bored it bigger for larger sizes. Same weight less horse.
Commerical vehicle engines and stationary equipment were the main uses. If you can find an old GMC pickup with one in it it'll most likely still run, they were bullet proof low rpm engines.
They actually made a V12 that was two V6 blocks made together and displaced 702 CI...used in fire trucks and rat rods.
If it was a GMC v8 from that era it would be a collector item and would be somewhere north of 600 cu in in size.....