Are they 63 inch springs? (I believe?) If so you need to do minor custom work, 20 gallon gas tanks on the long wheel base, single cab trucks wont work iirc, without putting axle not centered in wheel well
Also a half ton gmt 400 springs shoould NOT be used on a 74 c20.
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heres a 76 c20 (actually a trailer, but its a 76 c20 nonetheless) with smaller rear brakes (11 inch) and presumably a good but not largest spring pack possible
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heres a 78 k10
unless you are prepared to 1) use your truck like a halfton
2) be unbalanced to your front suspension
3) also do a LOT of work, new hangers, busings.. etc...