Which heads wpuld be better?

I think so. However, like you said, finding a junk yrad engine thats in great shape will take some serious hunting and luck. Thats why I said when you do a good short block as opposed to making use of a seasoned one or the even rarer junkyard jewel.
I don't think you'll need a bigmech. cam and 4,56's to get there, but a well exacuted plan you'll need.
Lightening up the car is a great way to help.