What rear is currently in the truck?
I don't know about your 55, but the later GMCs had Spicer rears which would not apply to the tech tip referenced, although a ring and pinion swap should be possible.
I'm not sure... but from the reading I've done it looks like it may be 5:14. I pulled the original split rims off of it... they sat at close to 34 inches tall, and put on a set of tires that were only 29 inches tall so I figured that made a pretty big difference. I replaced those with tires that were almost 32 inches tall and it did help but I'm still screaming at 45 MPH.
