Hi, I just bought a '65 Mustang coupe automatic transmission. According to the VIN it was originally a 6 cylinder engine but now has a 289. According to the badges on the front fenders its a high performance 289. How can I identify if it is actually a high performance engine? Is there a code somewhere on the engine? If so where?  

on the passenger side of the block, towards the fire wall above the starter there is a casting code. you will probably need to scuff it up with a wire brush and use a little mirror because im sure the exhaust manifold makes it hard to read.  This code will tell you the year.  It begins with C5AE or C6AE, for a 1965 or 1966 respectively.  The 289 high performance block (1963 - 1967) was the same as the 289 2 valve and 289 4 valve with the exception of wider, heavier main bearing caps.  There was no difference in the block whatsoever.  The make up was different, things like the crank were specialy marked when tested etc.... Hope that helps  

Thanks I'll take a look at that with my dad.  

no problem, if you need more help, let me know what the numbers read