I got a set of Autolite APP 104 Plugs shipped to me, for the second tune-up. The 4.6 L Truck came with Autolite AP764, and when I did my last tune-up, I used APP764. The APP764 are no longer made, and Autolite says the APP104 are the correct plugs. The APP764, have a 9mm long thread, and the APP104, have a 16mm long thread. I don't want to run the plugs in too far, for fear of the pistons hitting the plugs. How do I handle this? Are the threads tapered? I have a Torque Wrench, and Torqued the last set in, except for #8, there just wasn't room for the Torque wrench. Thanks.

 

You may be incorrect or do have some wrong plugs or information.
The APP 104 and the AP 764 have the same reach of .708" about 16mm, not a 9 and 16 mm difference.
The 764 has a supperessor in the plug to help control noise for some applications, otherwise "IF" the reasch is .708", both will run ok in the motor.
The motor has a tapper seat between the top of the threads and the plug body that is the seal against the head. The threads do not do the sealing.
Go to the Autilite website, call up the plug designations and verify what I am relaying to you.
You would never want to use a plug that projects into the cylinder very far unless the mfger say it will work for the application.
Nor would you want ro run one to short and exposed the threads on the head to carbon or corrosion.
Some of the reasons why you need to stay with the correct reach is the possible change in heat range, hitting the piston in some motor designs and possibility of a valve hitting the plug depending on combustion chamber design and lastly but very important is spark knock from a plug that is to long and collects carbon on the threads.
16mm is the correct 'reach, not 9mm.
These plug designs were also used in other Ford motors back in the 80s and 90s with appropiate heat range selections.
The discontinued plugs are for older applications that no longer sell.
Good luck.