This will ABSOLUTELY cause that knocking! I just replaced my "eared" FP with the stamped 1 piece (I got mine from Tee-Bird products for about $60) and it was like magic - i had been through the whole motor twice over the years, and I knew all the bearings/etc. were good, but I couldnt get rid of that knocking sound. new flex plate and all is great - silent (except for the music of solid lifters!)
Mine is in a '56 ford with a 292 and an air-cooled F/M, and the new flex plate bolted right up - no need for anything other than what tee-bird shipped me (some reinforcing rings for the center, but thats it)
As for the leaking, I highly doubt that it would cause a leaking rear main. The looseness of the rivets on the mounting ears is so very little, that I have hard time beleiving that it would have enough wiggle to cause a problem.
I only wish I would have known this when I rebuilt the car in 2002.... I would have spent the $60 for the part gladly to not have to chase the sound!
Ron Grove
Wauconda, IL