Opinions on what should be next

If you are going to keep the auto, get a torque converter, that would be #1 on my list.
Although if it was my car, i'd ditch the auto and put in a 5 speed. You can gain 1/2 a second from that alone.

you could get a better set of heads, something aluminum, such as the newer edelbrocks are some afr's, maybe a custom cam matched to your combo.

Without a supercharger or a 5 speed i think with the e7's you are going to have a tough time getting low 12's, you'd be more likely to see high 12's low 13's. Maybe even mid 13's. Guys in 5 speed lx's coupes have a tough time getting in the 12's with them, so i'm not so sure with a heavier car and an auto you are going to be able to go that fast. Probably is possible, i've seen alot of odd stuff happen with 5.0 mustangs, but i'd say it's unlikely.

Edit:i'm just noticing you have "nitrous" in your sig, what exactly are you running?

I think most tracks are lieniant with the 13.99 rule in a vert if you have a full interior. With the advent of the O3 cobra, everyone in a cobra vert would need a roll bar before they even hit the track.

You're probably mid to low 13 now, with the juice. A guy i know with the thumpers in a gt vert with a 5 speed ran 13.99, on street tires, so...

As i said, first thing i'd do it the converter, i hear you can get around .4 out of that alone.
I hear edge converters are good quality and fair priced, i've seen nothing but positive things said about them. Most of the other cheapos like TCI and B&M i don't hear good stuff about. Then you have the rest like level 10, precision and a couple others, but they are all like $1000 bucks.