If it's nicely done, it's well worth it.
400 was available in 79, but only with a manual trans. 403 Olds motor was what came with the automatics. That was it for the larger engines.
The Pontiac 455 will directly bolt into place of a 400. (or a 428, 350, 389, for that matter)
You can make loads of power and torque from one. Great street engines.
Yes, you could find a used Mustang for that price....but then you'd have a Mustang.
The last Trans Am made with the 455 was 1976.
And yes, it's the best-looking car of all time, to me.
The body lines and styling are great, the screaming chicken on the hood would have to go though, a nice metallic blue w/silver pin-striping would be perfect.