Rotors from a 1970 to 1985 K10 will be 6 lug x 5.5" bolt circle and should match his rear axle, but they won't fit his Mustang II
spindles. If the vender offers a 6 lug rotor or sleeves, and is willing to trade, that would be the least expensive way to go.

Eric
http://s1199.photobucket.com/user/sparky234/library/My%2057%20Chevy%20Stepside?sort=3&page=1
'57 Chevy 3100 (Little Red)
'81 RD350 LC - Fun, Fun, Fun
'83 GS1100E - Pocket Rocket