For the most part, GMCs from about the mid '50s - early '70s used Spicer/Dana rear diffs.
What you're really shooting for is a '68-'72 center section, preferrably from an autmatic truck. Here's a list of the axle codes;
'68 w/4.11 gear; OT, NA, NB, WI, OW, HW, JV; '69 4.11; HW, JU, NA, NB, OT, OW, OY, TF; '70 4.11; TAA, THU, THW, TJU, TLN, TMZ, TNA, TNB, TOT, TTF, TWI; '70 C30 4.11; TIE, TIH, TLJ, TLX, TLZ, TMR, TNZ, TRN; '71 C20 4.11; TAH, TAJ, TJA, TJB, TJP, TJS, 2G, 2J; C30, TAK, TAL, TAP, TPZ;
'72 C20; RAH, RAJ, RJA, RJB, RKK, RKL, RKS, RKT, RJP, RJS, RKB, RKC; C30; RAK, RAL, RAP, RJK.
I also have the #s for the rare 3.54 rear, RKH, RKJ, RKP, RKR, RKD, RKG.
