performance online question

[79 blue monster]

ok, im buying lowering spindles for my 3/4 ton gmc, and i am a little confused.

-On this page i click on the last one tittled 1971-87 CHEVY/GMC/C10/C20, 2.5" DROP SPINDLE SET
http://www.performanceonline.com/Spindles-c-1143/

-Next i go to select my year/model of vehicle, and it gives me these options
-1971-72
-1973-80 1.25" rotor
-1981-87 1.00" rotor
-1981-87 1.25" rotor
-1971-87 (C20) (+$171.00)

Now, my truck is a 1979 gmc 3/4 ton. So do i buy the "1973-80 1.25" rotor" or the "1971-87 (C20) (+$171.00)" ???

I really need to know because one is way more expensive than the other.