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The 7/8" hole you drilled may be a little too snug - I found that I had to go almost to 1" to get the proper fit on my lights when I installed them. This is one of the reasons I keep posting to get the 1 1/8" unibit so that you can go a little larger on the center hole. I found that when I opened up the center hole a bit more that the lights fit more flush. I also believe it will help to slightly shave/trim the foam gasket to fit once you open up the hole a little. Hope this helps.
I completely pulled off the foam gasket on the outer 2 lights when I put mine on. I also found out when putting my lights in that I didn't drill the 2 holes for the studs big enough the first time, and they wouldn't fit all the way down. I just had it drill to the next size on the unibit. Maybe your problem? But the gaskets definately fixed my problem on the outer 2 lights of the gap. Just make sure and use a lot of silicone.
I agree with Jeff, I suspect the holes for your studs are not big enough and the studs are catching, keeping the light from sitting flush. I wouldnt remove the foam backing, its there for a reason. We all have the same Ford lights and the same Ford trucks, if your lights are not sitting flush its something with the holes.