"somewhat rare"? that V6 was the standard GMC engine in the 60's .... having a perfectly good original engine in a GMC you're "totally restoring", why would you want to put a Chevy engine in it?
"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir "When we tug a single thing on an old truck, we find it falls off" ~ me Some TF series details & TF heater pics
