Versus E7 heads, the GT40Ps are an upgrade for sure. I set up my system on E7s and then used GT40Ps so the plug angle should work for either. You may wind up using a different style of plug wire if you build on the P heads and then switch to a differnt plug angle. I'm using taylor 409 90 degree plugwires for the P heads and they are just awesome.
For the turbocoupe turbos, since you have more displacement, you should seek out the larger of the three versions: the .60/.63 garretts that were on manual trans cars before 87.
97 Laser Red GT-248A Wingless model, Built Lincoln Mark VIII motor-Procharger D1SC, 3 core IC
2012 Black GT-Saddle, 3.73s, Brembos, Shaker1000