1979-95 Mustangs with factory 5.0 302 motors require a 7/16" head bolt when securing cylinder heads to the block. This includes some aftermarket cylinders that are not made specifically for a 351W setup.
1979-95 Mustangs running a 351W motor will require the use of 1/2" head bolts when securing cylinder heads to the block. The 351W motors featured 1/2" head bolts, unlike the smaller 7/16" bolts found on factory 79-95 302 5.0L motors.
Your factory 5.0 engine block takes a 7/16" head bolt. If you're looking to run 351W cylinder heads, you'll need a step down, or reduce, washer for these to work. Most of your aftermarket aluminum performance cylinder heads, like the GT40X head, also use a 351W size 1/2" head bolt hole. Step down washers take up the slack when installing a 1/2" size cylinder heads, on a factory 302 block that has 7/16" bolt holes. There are kits available with just the step down washers, as well as kits with the washers, and new head bolts!