you are making it harder than it has to be, we went through this in tech. you are going to a larger primary tube so you are giving up flow velocity for flow volume, this will drop off some of low end torque and give you hp on the top side(are you still with me). they say 1 5/8 are for more modded 5.0's and stay with 1 1/2 headers for stock/mild 5.0's but try and find 1 1/2 headers. I didn't notice it to much but I have alot of mods, okay maybe still mild
. going crazy over a set of shorties and thinking it's a big gain is nuts and spending huge dollars on shorties is the same nuts. the stock headers like I said over tech already gave the H.O 7-10hp over cast iron headers just the contours of the tubes suck. sooo get headers with a flange summit, ford racing , brothers, bbk are all going to do the same period!!! the only difference will be fit, some will go on nice some will be a pain. If you want a gain from headers and can deal with the tight fit and such then go long tubes, you get hp, torque improvements.
so to sum it up, you are going to lose low end torque..yes. you will gain upper range hp..yes. all the aftermarket unequal length headers do the same for gains, some are made crappy, fit crappy but still do the same.
here you go..
ford racing..fit the best, most people like them, no other issues or complaints.
bbk's ..alot of people on here use them, look good, chrome pits and gets rusty, some guys have had problems fitting to h-pipes, but most like them.
summit...good bang for the buck. no bad reviews yet. they stand behind thier stuff.
Brothers..don't know they just started making thier own headers.
MAC..individual flanges offer sealing problems, leak prone but still people like them. nio big fitting issues
JBA..good stuff, huge dollars.
pick a budjet, pick your headers.
stay away from flowtech, pace setter...junk..junk.
hope this helps.Best of luck to ya