I ordered four custom made springs for my '37 from Eaton Detroit Spring, with shipping it was around $500 per corner including hardware. You will need a whole bunch of measurements, weights, new spring hangers, shackles, bolts, etc...... Not sure about the GMC, but the Chevrolets have a cast front shackle holder in the front. What about steering, the stock gear box won't like the 4wd or a lift kit.
I have already modified the steering, toyota power steering
