Use 1 5/8 long tubes. A SB Ford does not make enough torgue to support a bigger tube or a collector larger than 3 inches. Install an X pipe also, best $ u could spend................
I would just get the 1 3/4. It'll save you money in the long run if you want to change anything later on. You really aren't going to lose a noticable amount of power right now. I'm suffering the 1 5/8 woes right now. The little headers are choking my engine.