OK, here is the whole arrangement:
68 Mustang convertible,
351W stroked to 427
Tremec TKO 600, Ford, .64 Overdrive, STREAMLINED CASE
QuickTime Bell housing, Perfect fit for Small Block Fords with TKO, TR3550, or T5 transmissions
McLeod Release Bearing, Hydraulic, Internal Bearing/Slave cylinder
That all being said, I found the headers below. Now I have to find an exhaust system that needs the least modification.
JBA Ceramic Coated Stainless Steel 1-3/4" Long Tube Headers for 1965, 1966, 1967, 1968, 1969, 1970, 1971, 1972 and 1973 Mustangs with a 260, 289, or 302 Engine and a Borgeson Power Steering Conversion Kit.
Look at JBA's website. Although mine is a '66 GT fastback with JBA's mid-length headers, the 2 1/2" exhaust kit they sell for my application required no modifications. If the system they sell for yours fits as well, you won't be disappointed.
I don't know how much modification you are interested in, but there isn't a whole lot of true bolt on convertible systems out there, least in my searching. 2.5" pipe will fit through the reinforcement area, 3" would be really fun.
This is a Pypes heavily modified 409 SS generic x pipe system /w Magnaflow mufflers to fit a '70 with a 393W / C6 combo, with shortie headers. This system isn't that bad cost wise, but if you do not know how to weld, it would get into you pocket book quite a bit. Quality hangers, V-band clamps and extra mandrel bends add up quick. This particular project has a pan-hard bar and through the valance 3" custom built tips, which makes for even more fitting fun with a leaf spring suspension.
You might want to contact a company like Magnaflow directly and specifically ask if they have a system that fits, or could be an easier modified system than going with the one size fits all approach.