I have a couple ideas but some have been covered. The filter and the line have been covered so I will go to the others. I would run a return style fuel system and Mallory has a wonderful 3 port return style regulator that works outstanding. Next I would get an ignition box that has a highspeed retard in it. The reason I use these type boxes is when you hit the nitrous the box can retard your timing at the same time, when you let off the nitrous the timing returns to let the motor run without nitrous at its optimum. All you have to do is wire the box and the nitrous system together and wala! I noticed your 60' times kept falling so it might be a slight traction problem as well. The converter might be going away, if it is not a strong converter it will start bending fins and start making less torque to the wheels. I have run nitrous Mopars since Marvin Miller was the secret system.