[Ant]

Having owned two Cobras, one I built with FE 390, and side pipes, it didnt really cause any problems on trip as the wind etc seem to overshadow the noise, unless you opened up the secondaries!

I am having built on this very race orientated Cobra and undercar exhaust as I cant stand sidepipes, I reckon they look like a hotrod, plus to get any reasonable muffles they need to be getting on to 5" diameter side pipes. Apparently the trick nowadays is with undercar exhausts, they dont use oval pipes, but a cones etc I guess speeding up and slowing down the air flow, I have a guy who builds exhausts for the Aussie Supercars, and its no problem to for him to build the headers and have two systems one for road and ...track!
He built a set for a friend who has a GT40, the mufflers are only as long as the transmission, and it is too quiet, and still goes like stink. I am not to sure how many pipes are going under the car, and I reckon he might start scratching his head when he finds how little ground clearance these cars have.

Go for the undercar exhaust, but try and avoid using a small diameter pipe over the rear axle, there must be better options than that, plus as mentioned maybe use two smaller diameter pipes under car, and locate muffler more to rear of car where there is more ground clearance!