Mystery Spark Plug Gaps

Looking at your query brings two things to mind. We have come across this before. I think you will find that the Bottom line on this one is that you definitely have Piston to plug contact at high RPM.
As to the cause of it, there is a chance that you have a slightly bent con rod in that cylinder.
There are several types of Pinto con rods. There is the early forged rod; these generally will not like much more than 6800 RPM with a standard piston.
The later type cast steel rod, found in almost all blocks with the 205 cast in it are suitable for 6900 RPM with standard pistons and with light weight pistons 7300RPM, anything above that and you should change them frequently.
They are all prone to stretching and if you have a slightly bent rod, which I think you might have, especially if you had a valve head hammered into the piston, then as inertia takes over (at high revs) it will stretch that little bit more than normal and dependant on the clearances in your head / block etc, the piston WILL hit the plug. If the new pistons are heavier than the old ones, then it will stretch the rod even more.
To cut down any further risks of engine damage, you really must check the straightness of all your rods as soon as possible. Brand new rods are available from Fords and does not cost too much. Worth finding out.
With the Pinto plugs, as you know, they have a taper seat on them and no plug washer, so you cannot add or remove washers to adjust their position in the chamber.
This is what you should do to lessen piston contact. Take the plugs out and look at them. Where the electrode comes from the body of the plug, mark the outside body of the plug with a thin painted line.
Now refit the plugs correctly and look for the line.
The line should always be as near as possible to the 12 o clock position. In this position, the electrode is not facing the piston, so there is nothing there to contact and as a thrown in benefit, the spark will not be shielded in the combustion chamber, so it will fire the mixture that little bit better. Now you will find that some plugs do not line up where they should be. When this happens, you should take them out and swap them around in the cylinders until they line up. If they still do not, then you will have to use another new plug and repeat the process until they do.
Once you have a good match on all the plugs, these are the cylinders that they should always be fitted to in the future.
This is a really good modification to do on any Pinto engine.
You should find that your misfire has gone after doing this, but like I already said, get those rods checked out.
The late ‘Pop C’