A 3/8" impact? Really? If you read the Ford TSB, it would seem that using an impact would be really bad. Not saying you are wrong, just saying it doesn't jibe at all with the Ford-recommended procedure for removing the plugs.
1974 F350 Wrecker 390/T18
1978 Ranchero GT 351M/C6
1991 F150 SuperCab 4x4 7.5L/ZF5
2006 Freestyle 3.0L/CVT