Hi Tim,
You won't have any problems with 500hp or 600 or 700 or more! The frame is amazing strong, so is the body. I am running a 428 dyno'd at 500 hp and other than the usual "getting things to fit" problems the build was fine. The only advice I have if you are going to use an FE is to lay the body on the frame and mark its final position before bolting in your motor. I ended up moving things around a little in order to get the headers to clear the foot box. It is also a tight fit if you are going to use a toploader. Other than that the build was great. The gelcoat only needed a little sanding to get a perfect paint job. Plus, its up to a half of an inch thick in some spots (very, very strong). As for 1/4 mile times, I haven't run any yet. It would vary car to car anyway depending on your cam, gears weight and other factors. Go luck on your Cobra search.
--Mike