The holes in my GM D60 are 5/8-11. I strongly encourage you to use studs rather than bolts. Also, I reccommend buying the D60 ubolt kit made by superlift and sold by 4wheel parts. Normally, I don't use either one but in this case it's a really good deal and it comes with studs that are coarse thread on the bottom to match the housing and then fine thread on top to match the other Ubolts. All this hardware is plenty long enough for the thickness of aftermarket springs.