Its terrible that I'm afraid to answer this based on an ongoing thread elsewhere.
But hey, no guts no glory right?
A set of 416 casting heads can usually be found cheap or free. If your willing to put in the time, and I'm not talking a couple hours, I had 20+ hours in the last set I did, and a lot more in the first pair as they were a learning experience. That being said, with a generous home porting and attention to detail, this can be a fun path to some decent heads. But your going to need the right tools, a compressor, and as I said, time.
If this isn't what your looking to do, then there are other budget oriented choices that may be better suited.
What year mustangs are you attempting to play with.
What's your budget to put something together. And not just motor budget, rear end, trans, converter, and finally motor.....
Vinnie and tech may be on the right track for something simple for you.
Longtubes, a decent exhaust, a set of 373's, a performer RPM intake, a decent 4brl Carb, and a plate kit.
Drives normal 99% of the time, you find your mustang and spray yourself a win.
It doesn't get any more budget then that.
If you have a little more money, then there are much better choices then Nitrous.
You get out what you put in. I personally love the home ported 305 heads, a 268 cam, longtubes, performer rpm, Holley 750dp, 2800+ stall and 411's.....that's a fun, dirtball cheap street combo that should let you hang with a lightly modded fox body. It shoud put a beating on a lightly modded sn95 as well.
Most people will have other opinions, and they are right for them. If time, tools, or skill for head work isn't there, then look at some offshore casting alum heads, still much much better then what u have. If a little more money is available, look at better heads and more cam, brodix, pro-filer, summit or jegs house brands. All work well, all "inexpensive"
Just my humble opinion.