I am very encouraged to hear that someone you know had some 260s that were quick. Sounds like you may have even seen the car(s) run?.
My research and the comments of others thus far are leading me in that very direction.....to have someone port/build the 260 heads with stock or close to stock size valves.
Bob Carrolls "Stockers Bible" says that 302 heads will fit a 260. It also says 351w heads will fit with valve train modifications and pop up pistons.
Too bad it does not say what the valve train modifications are. Pop ups are required to make up for the lost compression, but the head could be milled to compensate instead.
Just about any aftermarket head may just simply have valves that are too big for a 260. If they aren't too big, they are probably severely shrouded by the cylinder.
I would consult someone who builds ported Ford iron to find out what you can do. Then they could make a set of heads for you with valve sizes to fit the 260. Unless your going to build a full race 260, and maybe even if you are, a nicely ported set of Ford iron heads should meet the airflow requirements of a small V8 like a 260.
I used to work with a guy that raced 260 Falcons and a 260 Altered back in the 60s and he used Ford Iron back then. His 260s screamed.