well just for the sake of argument, I don't think you are making 300 hp yet, but you have a good foundation. I used to have a 306 with those heads but they were ported, an edelbrock performer intake, 65mm t.b, 1.72 rockers, b cam, shorty headers, off road pipe and it was making 335 at the fly. Finish it off with a cam, and a t.b and you should be above the 300 hp mark.
also, 93 cobras were rated at 235 (the 94-95 cobras were rated at 245) but they were also underrated, as many other mustangs have been.
Also, I agree with the tfs kit comment. Any alum. heads/cam/intake will take your gt40 combo. That was a great combo back in the day and a good entry level combo now, but will finish last to pretty much any other decent combo. Again, it is an entry level set of heads, so don't expect too much from em, especially unported.
One last thing, you said his "dyno sheet said it was making 270"?? if that is the case then the dyno was most likely reading it from the wheels unless it was run out of the car which I doubt. over 300hp on a light car like that is pretty impressive.