Unless you run a class that requires you to use the E5 heads, I certainly wouldn`t! They have 69cc combustion chambers, so very low compression (my 85 Mustang is only 8.4.1 comp. with these heads, with factory flat top pistons). They also have a much bigger exh. port bump than the later heads.In fact, they only differance between the D8 302 2barrel head used on the 139-145 HP Fairmont & Mustang engines was a minor change to allow the new-for 85 hyd. roller lifters to be removeable without removing the heads.The 69-70 351W heads are good, as they have a bit bigger valve size, & smaller CC`s.