the cat to res bolts you can use a 1 1/2 long bolts and nuts and like 1/4 inch wide threads will work. those are gonna break. count on it.

if the top bolts break you'll need to drill out the old stud and then nut and bolt the new DP in up there.. not sure how big they should be tho.

get the engine good and hot, then try breaking the bolts free can sometimes help with stuck bolts. the heat frees the rust some and makes the nut expand.