I know the Chevy & GMC trucks share the same parts in the '47-55 1st series AD trucks: Frames[except the front cross member ] Cabs,bumpers, doors, fenders [ + a light hole ] , running boards,axles, brakes, transmissions,beds, driveline & lots of accessories. I
I know the GMC does not share the same engine,grill, hood & tailgate & horn button.

What did I miss on these lists ??

Front bumper splash pan and genral motors script on speaker grill (but I'm sure you new those). That is all I could think of.

You did a good job mothertrucker, can't think of much else myself, as far as light duty Chevy & GMC Advance Design trucks are concerned. I don't have a light duty GMC right now, but I do have Med. & Heavy Duty Chevy & GMC's as you know. The larger trucks have some of the same small differences, But I have noticed that in a few places instead of using rivits, the Jimmys used nuts & bolts, & the frames are heavier. Like I said in another thread, the Canadian GMC's used Chevy engine & drive trains, & the '52 &'53 larger GMC's used the larger Med. duty Chevy front bumpers. The Heavy Duty small cab GMC's used the same cab as the Chevy, but used similar but different fenders. The large cab AD GMC's are a different animal altogether. Spanky

More differences between Chevy & GMC Advance Design trucks: Tailgate scripe stated " GMC " Hubcaps are different. Oil pressure gauge tops out at 60 lbs [ not 30 as in the Chevs ]. Radio dial reads " GMC ". Hood ornaments were different. Radiator support & surrounding sheetmetal were different than Chevy.
You may notice that GMC trucks are absent of "clutch head" screws. Every GMC truck I have parted out was absent of " clutch head " hardware.
Almost forgot.... all GMC electrical systems were Positive ground systems.

MT do you have info as to when the GMC went to a Negative ground system, I was told by a generator rebuilder today that the generator don't care whether it is N or P ground just the regulator. is that your view also?

You are right Mack. Generator cranks out voltage & it dont care about polarity. As far as I know The GMC's went to 12 volt neg. ground with the introduction of the Task Force design trucks in late '55. Maybe the " Kook " can help out here if I am wrong.

Thought of another difference, MT, didn't the early (Advance Design & back) GMC's have a long bed 1/2 ton.

Spankey ,you are an astute observer for sure........ Yes GMC wuz the only only one to offer a 1/2 ton longbed. I commend your bad self for reminding me of this techincality.

This comment is a little late, but I will add it anyway. 1947-1948 GMC 1/2, 3/4 and 1 ton trucks used longer frame rails that ran almost out to the bumper bar. The longer rails were required with the heavy duty frame braced grilles that were used on those 2 year models

Warren , the 1 ton Chevy truck used a H.D. mounting & was specific to the 1 ton trucks only.[ different than the 1/2 & 3/4 ton trucks ] Are you saying the GMC's used this same H.D. set up on the 1/2, 3/4 & 1 ton GMC's for 47& 48 ??

I have parted out over 100 1947-55 1st series AD trucks & I never saw a 47-48 GMC truck with longer frame rtails up front in the 1/2 ., 3/4 & 1 ton trucks.

Thought of something else. The '47 & '48 GMC's had those different, more rounded front bumpers, & the '47-'51 GMC's Dash Guages had a cream color background,with maroon letters & numerals, compared to the Chevrolets dark brown background with beige letters & numerals.