Nice kill.
Those style mustangs were complete dog sh!t. I once raced a 94 GT auto vert in my 1st GTP and all I had at the time was a open cone like you and mufflers. I literally put a couple buses on it from 0 to 90.
I also once ran a 95 auto GT w/ intake and exhaust, in a buddy's bone stock 97 GTP with 87 octane in the tank. Put about 4 cars on it through 1st and slowly kept pulling away until over 100. I bet a stock GP GT could take one of them if the driver isn't on their A game. They are just painfully slow.