Brendan - A quick check in the Chevrolet Master Parts Books shows the 1940 Chevrolet 1/2 ton (KC) and 3/4 ton w/11" rear brakes (KD) shared the same rear axle parking brake components. The 1940 3/4ton "Special" (Heavy Duty) pickup model KE w/14" rear brakes shares the Huck parking brake parts with 1937-52 1 Ton models, so not the same.
1940 GMC 1/2ton (Model AC100) and 3/4 ton (Model AC150) shows both used the same parking brake pieces in each truck too in the GMC Master Parts Books I have.
One step further - I compared the GM part numbers in the Master Parts Books for both Chevy and GMC and 1940 Chevrolet 1/2 & 3/4 ton pickups and the same sized 1940 GMC pickups are the same. *(See the list below I posted previously in a Swap Meet listing which I've added the GMC Parts Group#s to)
So yes, they will fit your truck!

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Chevy/GMC Parts Group - Description - Part# - Qty **
5.149/4-88 - Lever parking brake shoe 8" OL - Left GM# 475477 - 1 each
5.149/4-88 - Lever parking brake shoe 8" OL - Right GM# 475468 - 1 each
5.150/4-88 - Extension-strut, parking brake shoe lever 7-1/16" OL - GM# 475481 - 1 each
5.150/4-88 - Extension-strut, parking brake shoe lever 7-1/16" OL - GM# 475482 - 1 each
5.151/4-88 - Spring, parking brake shoe lever extension 15/32" ID x 1-3/8" OL - GM# 476104 - 2 each
5.158/4-88 - Bolt, hex head 3/8" - 24 x 15/16" w/eccentric - GM# 475492 - 2 each
8.917/4-88 - Nut, hex head 3/8" x 24 - GM# 120369 - 2 each
8.932/4-88 - Lock washer, internal tooth 3/8" - GM# 138557 - 2 each
**Added GMC parts Group #s after the Chevy #s - The GMC info is taken from GMC Master Parts Book X-145001 dated 7-5-1945 with changes 1-5, and Supplement X-145020 dated 12-1-1945.
PS - If they are still there, the clips between the parking brake cable ends and lever will fit too!