I just replaced the cat-back exhaust on my '87 w/ 300. Muffler was 2.25 INSIDE diameter, both ends. Advance Auto was having their aniversary celebration, 20% off every thing purchased online, free shipping & and a $50 coupon if more than $125 was spent. So I bought new extension pipe, 31" Cherry Bomb, Tail pipe, new hanger rubbers, upper & lower radiator hoses, thermostat, cap & rotor, Plugs, Plug wires and fog lights. Total bill was around $155, after sale price reduction & coupon, my total was $83.