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I have run Blackjack headers ($100 back then) with 1-5/8" primaries (maybe even 1-1/2" don't recall back then), and had a huge performance increase with just a mild cam and vac sec holly 600cfm carb. Later I stepped up to 1-3/4" primary Hooker Super Comps, with no performance increase.

So my moral is, if you go headers, keep the primaries small unless you are running aftermarket heads. You will retain a lot of torque and it will sound great. Ground clearance should not be an issue as the collector is pretty high of the ground at stock height so 2.5" shouldn't be killer. BTW, dual 3" pipes with Flowmasters sounded great albeit a tad loud.