Hello Brian:
Although I'm no longer active with the
Vintage Mustang Forum, their
'Parts for Sale' &
'Wanted Ads' might help you find the needed exhaust manifold.
Of course, I'm assuming that the 428 powered Mustang used the same stock manifold as your Park Lane. This may or may not be true.
I found the folks on VMF to be friendly & helpful so they can steer you in the right direction.
Good Luck,
Paul