You can notice the increase in torque but the changeover is not complete so no really ran it much. Still need to replace the carbs with the injection system, the engine has no distributor so is running at a fixed 10 BTDC so is not good revving out plus the exhaust needs doing.
I haven't seen those headers but most likely they will be for a 3.5 so a bit small for the 4.6 - the P38 exhaust is a larger diameter.
Cheers
Garry