Unit the X heads are actually called X302 but are abbreviated at GT40X. The Gt40Y's are really Y302's. As the very old J's are J302's. Thats what she means. The 303 is the cam specs.
Skyler
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-1989 Saleen Mustang #406- TFS Heads, E-303, edelbrock intake,70MM TB, 73mm MAF, off road H, 2 1/4" exhaust, and 4.10's.
332RWHP@5500
351RWTQ@3750
Runs on 87 Octane!