didn't GMC have an open drive in 54 like Chevy did?
any 4 speed won't "improve cruising speed", no matter what rear gears you have, both 4 speeds you're talking about [and a standard 3 speed] are 1:1 top gear .... you can get 3.55 gears for the original carrier in your 54 axle which would mean you don't need an OD, but stock it would be a 3.90, which as crenwelge says is best for an overdrive, even if you put in a modern one like a T5 or automatic 4 speed where 4th is OD
Bill
