You need duels or 3" single at least, and if you don't want to headers the best flow and fit is 92-3 magnum manifold, fit perfect. On one of my trucks I had roll the firewall/floorpan seam a little for clearance, duels on my 77, right one runs straight back, left one crosses under front of trans, parallel to the right one, mufflers vertical near stock location, over axle then off to the left, both pipes exit under bumper, balance tube just a head of the mufflers, pulls hard to 6500 with my 408