College is a good thing, and often needs to be treated as a priority. So far I've paid for four degrees and may have numbers five and six still to deal with. I only did the course work myself for my own two degrees.
I expected to hear that its a 100, a 150, a 250, 280, 350, 450, something like that. The serial number would answer the question if numbers on the hood do not.
1st Series usually means a '55 Chevrolet, and strictly speaking does not identifiy any GMC. Having said that, if you look at my signature line I identify two of my GMC's as '55 1st. What that means is the trucks were both made after the change over from '54 models to '55, but before the change to the later body style. Chevrolet makes that distinction. I make that distinction. GMC did not, and GMC purist all think I'm confused.
The series or model information I'm asking for tells us what load rating range the truck was built for. The serial number found on the plate in the driver's door opening will clear it up.
A '53 100 (1/2-ton) will have a torque tube.
A '53 150 (3/4-ton) will have ?
A '53 250 will have a two-piece open drive shaft.
I've seen 3/4-tons with partial torque tube, partial open drive shaft. I know after '54, all the trucks were completely open, but in 3/4-tons it might have happened earlier, and it did in 1-tons.
Someone who knows 3/4-ton trucks better than I can jump in and clear that up. I've only owned 1 3/4-ton ever (ignoring the modern one I have now), and it was a '47. It had half torque tube and half open.
Anything bigger than a 250 will have a multi-piece open drive shaft.
Just for sake of example I'm going to assume you have a 1/2-ton truck, which will be a 100-series. 100 on the hood and a model number in the low 100's.
My '55 1st GMC Suburban is a 1/2-ton truck with the GMC 270 motor, an original type 4-speed and a later rear axle with 3.08 gears. I can run 75 on the freeway with out straining the motor and its quite comfortable at 55.
There is more than one Saginaw transmission out there. The one often swapped into these trucks is a car type 4-speed. Its going to drive like a car and have the same 1 to 1 final ratio as the truck transmission I have. Choosing your rear gears and transmission need to be done so they compliment each other.
Another popular choice is a T-5 5-speed, which provides an overdrive in 5th and means gears closer to 4 to 1 might make good sense in the rear.
I hope this additional info helps you answer our questions so we can do better pointing you in the right direction.
