I have speed engineering 1 7/8 on my denali 6.2. I would say, if you intend to break 450 hp, buy the bigger ones. If you are unsure, buy the bigger ones. If you won't come close, buy the smaller ones. But get a y pipe from them too. Getting an ORY made is a huge pain. In fact, I'm a shit welder. I have no experiance with exhaust. But the only way I could use my headers was to build my own y pipe. I wanted manderal bent 3" to a 3.5" collector to use my stock 3.5 exhaust (you should have 3") this took me almost 200$ in manderal bent pipe and almost 2 weeks working on it 2 hours per day. And I'm not exactly happy with how it turned out. It is a true 3 to 3.5 y pipe and it is all stainless and doesn't leak. But if I had a choice (like you do) I would have got the speed engineering y pipe. Exhaust shops wouldn't do it because I didn't want cats. Some legal crap.