Any service manual will get you by, but there is really no need buying one unless you get it for your year. Some difference exist, and while it might work for the engine, other things could be amiss if you tried to refer to it later.
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'13 Mustang 3.7V6 - MT82
'00 GP SE (L67 Swap, XP, Headers, 3.25")
'98 GTP, '01 Bonne