I think the major problem regarding rebuilding of the Ford 4cam Indy engine is that it is very old in terms of years, although obviously this technology is not obsolete. Not that many were made, parts are very hard to get, and they are very expensive. And, they are relatively small engines (256 cu in) so the amount of power you can pull out of them is limited somewhat by displacement. Also, modern valve train technology has improved so that pushrod engines can rev higher than in the past.
Lots of power can be made from relatively small engines. I had a hair-raising ride in a Ferrari 288 GTO today- all that power (400hp) from 170 cu in!!!!!! What was really impressive was how smooth and tractable the engine was, and how flat the torque curve. But it took twin turbos, intercoolers, and electronic fuel injection to do this. And, in 1985, that engine was, I think, state of the art. I think you could get a lot of horses from the 4cam Ford vintage Indy engine- but I can't think of a more expensive way to go about it.