I used to have a 90' T-Bird SC that did the same thing. We drove it back from Panama City to Chicago for Spring Break 93' or 94' (ah the good old days) , lights flickering most of the way. It was the switch, at least thats what the dealer told me.(waranty).
Im sure the fenders will interchange, but I think you cannot fit as wide of a tire in the front with the 87 to 90 fenders, or was it 89. Bumper covers should be the same. I do not have proof of my answer, but Im sure someone else will chime in.
Im in the Chicagoland area, and have not had a plate on the front of my car since I have owned her (2.5 years ago). I have had one ticket for no front plate, and 2 for tinted windows. The price to pay (I think $20.00 a ticket) to look good. :D
I Have lowering springs also, (FMS C springs). I think the exhaust, and drag radials helped more than the gears for the track.
By the way larger gears are better when stuck in traffic, less clutch work. the 3:27's were horrible.
I go to the track about 3-4 times a year, and love my 4:10's. I don't see that many Cobras (96-98) with 4:30's running much faster than with the 4:10's. Maybe I'm wrong though. :shrug:
Changing the gears in your Cobra would have no effect in messing with the computer in your car (just make higher RPM's on the highway). As for your surging idle, try a search on IAC valves, sounds like this may be a possible problem.