Best mods for immediate improvement?

[Mr 5 0]

After the gears and exhaust, go for at least a shift kit for your AOD, bump timing to 14 degrees, a decent fenderwell cold air kit and of course, a total tune up including new plugs, cap/rotor, new hi-po wires, gas filter, etc.

A 409 in a '55 Chevy is probably a 13-second car but every set-up is different and it could be a 12-second car - or a 14-second car. Depends on the 409's mods as well as driver skills.

You really need better heads and intake to get into the 13's but gears, exhaust and tweaking what you have will give you a solid 14-second Mustang and it might be enough. Gotta start somewhere.