Pretty open question and many didn't even stick with Ford.
It's been a while but the last Model A Ford dad rebuilt was only about $500. With the high compression Ford racing head on it, I think it was around 50 hp, which gets close to the $10/hp mark.
Then there are a half dozen flat head V8, the Y block, and various other inline 4s and 6s, along with some v6s, that were not considered.
If we are talking 400 - 600 hp range that someone would be willing to put in a Cobra, for the price of decent heads and a cam, a 460 would go well over the 400 hp mark. Its an easy engine to work on. A person could pull one out of junk yard and get there for about $3500 or so. That would be hard to beat.
2nd thought:
You could get over the 400 hp mark with a 351 W the same way and maybe less cost. However, on the 460 you could use factory heads. Port match intake and exhaust ports to the gaskets and clean up the rest of the ports a bit, while your at it. I think you could easily get over 400 hp for less than $2K.