1 minor point: cyl 5 is not the driver front; it would be one of the middle ones.
Are you really sure about the rings? When that plug blew out, some particles could have fallen into the cylinder and damaged the rings, piston, and/or cylinder wall. For as bad as your issue is now, it would have been fairly evident if the cylinder was at least visually inspected before the 'new' heads were put on.
I'm not very up on OHC engines, but what about the cam itself? Would a bad lobe do this?
To get at the root of your problem, most likely the head is going to have to come back off. Good Luck.
04 Mustang GT vert basically stock
88 Dodge Ramcharger 4X4 for those things a Mustang just can't (or shouldn't?) do.
98 Ford Taurus SE Comfort --- wify's ride.