There's been serious tests done on different manifolds vs headers, and as I remember, "at about" the 300hp level, there is no significant difference between headers and manifolds, certainly not with a 273 and the trouble you have to go through
He does a VERY professional job and is a stand up guy. The Magnum manifolds come stock with 2.125" outlets. Plenty big enough and since they are Magnum manifolds, they flow very free. Plus he does all his tricks to them and makes them even better. Read the thread. He is a good dude.