PTA does not use the plate, only the fixed rear shocks, springs and spoiler of chassis 1.
Crank the heat up, which will make for less HP. 150 F and clear weather should bring the PTA HP down to around 470 HP and since you'll have to run 0 tape to keep the water from boiling, it will add drag. If they are still too fast, lower the tire pressures which will increase drag/fricton.
The in game, on screen method of changing the weather will only let you go to 110 F so you'll have to get into the track.ini and change the weather there (back up the original first) and then run 'realistic' weather in game.
The trouble now is your tires are going to get really hot so you may have to adjust the track.ini 'track_tire_heat' to keep them from melting.
Have fun!