grab a 3/8 and 5/16 POS bolts from home depot and try. I can't see it being a different one. My 1 ton blazer uses 3/8 front and rear. My last leaf pack (off an 89) used a metric size, can't remember the size, like a 10mm or something
I have heard of ProComp lift springs using 5/16, which I can't understand why they would go smaller.