The Stovebolt Site Forums The Parking Lot Big Bolts 56 GMC 1.5 ton: 6 or 12 volt?

It should be a 12v. The last 6v GMCs were the '55.2 I6 models.

Of course, if it has an I6, and was mistitled as a '56 (sometimes they were titled by year of sale, not model year), it could be 6v. Check the body number at the website in my signature.