you'll have clearance issues if you've lowered the car mainly.
Depending on what you get for the LT1 the headers may actually hang lower than standard. I lucked out with some ebay specials from the states that we old stock kooks or similar and they fit well and only cost a couple of hundred quid.
I've got a custom true dual system on mine, my dad's an engineer so we measured everything up and made a custom system. Although its' very loud (as I wanted)
Bang for buck the custom exhaust places usually have reasonable prices and as long as they do the over axle bend correct it will be nice, and should only be a few hundred quid or so..
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'99 Z28 A4, 253 LS2 heads, 228R TSP cam, Patriol Dual Springs, Double roller timing chain, LS6 ported oil pump, OBX LTs, 3" True Duals, LS6 intake, 42lb/hr injectors, SLP lid + bellow, EFIlive COS 3 Speed Denisty Tune (self tuned), 3200 Circle D stall, CAI mod, KYB AGXs, Eibach Pro Front Springs, UMI panhard, LCA + Brackets, Strut Brace, Custom Subframe connectors, WS6 axle with 3.42s, 18" American Racing Rebels, US mirrors, 93 tail lights, 3.5 inch custom cowl hood. Mickey T's Street DRs on Vette-ZR1 wheels for the strip !
Best ET: 12.0 @ 116 mph (on a 0-60ft of 1.8 s)