I've never seen any GMC Stepsides,but I've seen a lot of Fendersides
I don't know if I ever saw a big block C2500 Fenderside before. I've had two Chevy C/20 396 Stepsides,both '69s. One was a highly optioned CST/20,which I thought was rare. Someone wanted it just that way,Medium Gold Poly w/Gold (Sandalwood) interior,A/C,Auto... The other was cool,too. It was a Woodland Green C/20 w/Green interior,4spd,No-Spin,clearance lamps,HD everything,accessory spot light and grille guard. It had 8.25 x 16.5s on front and 9.75s on rear painted black w/o caps. Someone knew just what they wanted with that one,too.
I've seen plenty of C2500 and long 1500 Fendersides over the years. Just can't say what engines they had. I have a K2500 and I'm putting together everything for a Fenderside bed now.