Bolting them on the block is no issue. The headers probably will not work. I think the Ps require a special header due to the spark plug angle. Hopefully, someone who run these will chime in for a final verdict.
I have the Gt 40p's on my 68 w/302 and yes they require a special type of header. I bought a set of MAC longtubes that clear the plugs and everything else overall a nice fit, on another note these headers use a ball socket type joint vs the 3bolt flange. There are also shortie version headers for the P heads that will fit and clear.
There are also tri-y headers that will fit the P heads. Some of the mustang parts places have them. Disadvantage is that the collector is only 2.5 inches and the pitman arm will hit one of the primaries on a hard turn.