big block a bodys and clearence seem to go hand and hand!!!! what ever the case i'll get this 73 400 rummbling or die trying!!!! maybe i'll have to fab some bump stops to keep the axel from hitting the pipes.
You should be fine. I don't know what headers I used. I bought the car dissasembled with lots of part. and I couldn't find a part number on them if my lift depended on it. underchassi headers are expensive, but you can use B body header with some simple mods ( a hammer and dents/dings) if you buy 2.5 inch exhaust, you should be fine without bottoming out. you can put 2.5 inch on a SB, and the rear is the same as the BB, so I don't see it being a prob.
You could buy some B body headers for $150-$250, and buy 2.5 inch exhaust kit ( includes pipes, clamps, mufflers, hangers) $199