I'm doing an undercar exhaust on a 289 right now. It has the CR drag headers. I cut off the two 2" elbows turning out and replaced them with 1 3/4 going back as that is all you can fit between frame rails and footboxes. Then a 2 into 1 with a 2" exhaust. I'd link to CR but there catalog isn't working right now. I've used Thrush oval mufflers from Summit as they are very flat and barely hang below the frame rail at only 3 1/4 deep.
http://www.summitracing.com/parts/WLK-17704/ The 2" is more than adequate exhaust with twin pipes on a small block. The centre in muffler gets the 2" under the cowl hoop just outside the gusset and it's slightly above the frame. It's coming together really well right now and I can't wait to hear what it sounds like. I'll post some pics when I'm back on the shop computer.