That '56 Tandem is the one, out of all pictured to have. It's a Heavy Duty 10400 series with an 6 spd. automatic trans, aux. tranny, & tandem axle. If it still has the original engine, it will have the big 322ci. Buick V8 engine used in the '56, & '57 Heavies. Probably air over hydraulic brakes(note the air horn on top)."RARE" truck. There's a member on this website( BHMOTOMACH ), who's father has one like it restored. See it here,
http://photobucket.com/1956_chevrolet_tandem
