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5.0L (1979-1995) MustangTechnical discussions on 5.0 Liter Mustangs within. This does not include the 5.0 from the 2011 Mustang GT. That information is in the 2005-1011 section.
GT-40P are iron heads. GT-40X and Y are aluminum heads, Y just being the smaller sized of the two. The GT-40X come in 64 and 58cc. I put my GT-40X 64cc aluminum heads on about a month ago during an easy 12 hour H/C/I swap. Feels like a different car.
Cheapest I've found them new is $989 for a pair (assembled).