Steven,
The closest version GMC produced to your 2 axle 4x4 Chevy G-506 was the 3 axle 6x6 G-508 CCKW.
The engines originally installed in GMC g-508's were 270's, but I am not certain for AFKX's. Just go slow trying to free-up the 270. Apply copious amounts of the rust penetrant of choice and practice patience before bringing out your hammer collection. Good luck.
FYI, 302 engines were original in Korean War era GMC M-135 and M-211 6x6 trucks.
Paul
