I may have already replied to this but I don't think you're going to see 20 horsepower going from a header to a slightly bigger header. Especially with 3" exh already, i look else where.
When I was building my 408 Ryan Johnson freshened up my Edelbrock heads. He and I were talking on the phone about the build and I mentioned headers and that all I had were a set of 1 5/8- 1 3/4" TTI's off my 340 but was considering their 1 3/4" "race" header. He told me to buy a set of Hedman 1 7/8" with the 3 1/2" collector. Ryan said on builds similar to mine that he had seen over 30HP gain on the dyno over the 1 3/4" TTI's. I bought them and haven't needed a better header yet, I have wondered if a 2" Hedman would be better on the 434 though.
Good headers are expensive, IIRC my Hedmans were around $650, plus another $350 or so to ceramic coat them inside and out. Building engines that make some steam are expensive, so why use a cheap header or one that will hold it back?
Just my thoughts, this and $1.75 will buy you a cup of coffee in some places.
