91 z28 350tpi 145k stock bottom end, stock heads, lt4 hot cam, lt1 intake swap, t5 and 4.10's

Go to the track and get some numbers!

Honestly, it's tough to say if I were to guess with no dyno numbers or anything. I'd say with the stock heads holding you back and if you hooked nicely, you should do upper 12's to low 13's. Most likely very low 13's.

Yea thats what I'm hoping for. heads just arent in my budget right now. i made 238rwhp and 338 rwtq with 1.6 rockers, shorties, t5 and 3.73s stock cam and intake

I made 284 RWHP in my old 08 mustang which was heavier than a third gen and I ran a 13.5 @ 103 mph. You should def do at least around a 13.0 with good traction. My tires at the time were junk too and I could have def gone a few tenths quicker with good rubber. Get some good tires on your car and I can see high 12's.

My friend has some 26" slicks he said i could use the next time we go to the track.

mid 13's. a local car here goes 13.6 through a 700r4 with ported heads and a hotcam kit... stock stall is killing him.

what cam and gears?

One of the LT4 hotcams, but they make two of them. One is like .495" and the other is around .525"... don't know which one he was running. I didn't talk to him too much about it cause I was on a date so I don't know what gears. I do recall him saying the heads were ported though. Only other thing I noticed was headers.
Oh and he had 2 10" subs and he was a tad bit chunky, so you can probably knock another 1/10th off that.

OP has a manual trans car though....Doesn't take much power to get high 12's as long as you have a tire and suspension (and driver mod) to back it up. There were guys making 290 RWHP in 05+ mustang GT's hitting 12.8's or so around me. And that is in a 3600 lbs car!

With a very experienced driver, yes. That's comparible power/weight of a 4th gen and I know they have broken 12.8's with just tires.
Average driver... no way.

I had a motor we figured to be around 310 horses at the crank (so around 270 rear wheel), couldn't break out of the low 14's power shifting. First time running a stick car at the track though.