Based on Bill's research, I did some diving through through both GMC and Chevy Master Parts Books and the web. Both Chevy and GMC list the same front wheel brake cylinders for the 3100 1/2 ton 4 wheel drive front axle for 1958-59 as Bill indicates (note part numbers have some transposed numbers). Chevy actually lists them as 1957-59.
The descriptions of those pieces show they are one inner diameter - 1-1/16" on both ends, and not the stepped two different diameter wheel cylinders used in the earlier forward drive axle assemblies.
More diving through the parts books show that GM#'s 3749126 and 3749125 are listed as sourced from BP (Bendix Products) Part Numbers 49455 and 49454 respectively.
I checked an old NAPA Brake Parts catalog to see what it listed, but only found it listing wheel cylinder repair or overhaul kits, NAPA#s "20" and "20H respectively. Those kits confirm that the cylinders do use a single 1-1/16" bore to match the GM data.
Using the original part numbers and Bendix numbers, I searched a parts history listing for US Gov parts and came up with the following cross referenced manufacturers and part numbers that it list as replacements for the original GM part numbers (these are specific to GM# 3749126 only) :
Bendix 33462
Allied 3463
Ferd C5TZ-2061-A
Ferd C30Z2061C
These numbers in turn cross reference to other currently available wheel cylinders, like this
set from Amazon. Or you can get them separately as:
Dorman
W41718 LH Front wheel cylinder - 1-1/16" bore (this part was previously listed as WC13461)
Dorman
W41719 RH Front wheel cylinder - 1-1/16" bore (this part was previously listed as WC13462)*
Both of these are relatively inexpensive and you should be able to acquire and test fit to be sure they will work on your backing plates.
Best of luck Steveoreno, and please update on findings/results!
* Edit Note = Corrected my part number typo from w47719 to W41719
