62' short box rear drop question

[IHutch5R]

I'm lowering the rear end on my 62' C10 swb fleet side. I've already got the front where I wantit. My question is on rear springs.. im looking at the 4" ones that brothers trucks has and the 4" from CPP. Does anybody know if one sits lower than the other? I've seen that on other cars where one brand is lower than the other even though they say the same inch drop. Also, I've been told to get the heavy duty CPP springs vs. the regular if I go with them.. any knowledge on that would be great. Thanks