Actually, you may have to re-adjust your expectations. We typically see the "best of the best" when it comes to ET, 60 ft, 1/8 and 1/4 mile times posted. As it concerns a very high percentage of these cars, many would be happy with a 12.8 quarter. But you'll rarely hear them post that!
Lets look at a fairly common replica as an example. An FFR with the classic 302, 5 speed, live axle and something like Goodrich TA's or Goodyear GT-11 tires. Now let's put a "decent" after market cam in the 302, some nice AFR heads and a proper single four barrel. Or you can tweak your EFI if you prefer, no matter. Plenty folk be happy to break into the 12's, not kidding, with such a setup.
OK, out with the 302, in with a 351W, another common setup. That will likely get into the 12's, mid, maybe even low 12's if it's really good. Tires are a HUGE factor here, better step up to something with some bite. Avon's maybe ($400 a tire, not gonna happen with our budget Cobra). Goodyear bill boards are a good choice, but controversial and big enough they might not fit without "jacking up" the rear or something like it, not good. Tires remain a conondrum on what to do!
Want to get in the 11's? Aint gonna happen without figuring out the tire thing first, you WILL have to step up to something. The 302 or 351 will need a SERIOUS build as well, a few bolt on after market parts aint gonna cut it.
I'm running a side oiler, high riser heads, dual fours, Goodyear Bill Boards (about the best you can get for street/strip traction without going to slicks). My "race motor" dynoed WELLL OVER 600 horse, 7500 rpm limit, nasty as you can get. Solid roller cam, 12.5 to 1 compression, race gas was a MUST have. Drag race carbs (6 mpg), barely streetable. Sitting down? It ran 11.90, on the Goodyears, traction challenged all the way. 1/8 mile was right at 8 seconds for a 100 mph.
Rebuilt the motor, dumped that race cam, went with a "lumpy idle" flat tappet, 6500 rpm. Reduced compression to 9.6 to 1. Threw out the drag race carbs, went vacuum secondary smaller duals. Never dynoed after ward, I figure I gave up a good 150 horse. Ran the 1/4 in 11.99, doubled my mpg from 6 to 12 (actually better on the freeway), WAY more "streetable", pleasant daily driver now. Cannot FEEL the difference in power, I just can't burn rubber as far as I used to.
It's all about the tires! Which is why I just mounted some Goodyear slicks, hand grooved for the street (still illegal as hell I guess), but whatever.... I think I CAN DO 11.90 again, maybe much better even! With the "street" motor, now that I got some decent rubber!
12.8? Not bad, I'm impressed.