I have ‘41 to ‘47.1 GMC trucks. 1.5 ton and larger have 20” wheels and the tire cutout is larger than smaller trucks. GM also made rare full width fenders that fit 20” tires(I have two sets) allowing a stock 4wd full width front axle to be used as well as large tires. Did you perhaps get one of these fenders? I have pictures with dimensions if you wish.

Ed

'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.