gt40 vs gt40p

[triple-aught]

I have a '66 289 2v car and planning to do a stock rebuild on the unmolested eng. the only upgrades now are a holly 2v 350cfm man choke, true duel 2 1/4'' into flowmasters, a msd master blaster coil+ wires, and a k&n air cleaner. Kinda like a stage 1 harley upgrade for a ford. I want to keep it reliable and long lasting more than power but would like to take advantage of the better breathing. I was looking at gt-40 heads as a mild upgrade over the stock thermactor heads (i removed the smog pump and planned to plug the heads when rebulit, temp caped at the valve). but read on this thread that headers are needed? I'm looking at $800 to rebuild the stock heads including hardend valve seats i want to keep the stock man cause of the new welded exh. what does any one recommend for heads and cam? I up graded my 2000 harley twin cam with S&S e carb, opened up the exh. and S&S .510 gear to gear cams (ditch them timing chanes) the next bump up from stock (.479 i think?) and love it! it's reliable, fast and have greatly expanded the powerband. I want that for my car + can anyone tell me what is the stock lift/dur on the stock cam? the car is automatic