As a fellow 69 grande owner, I also looked into this. With some work, a 460 or 460 based stroker can also work if you want some serious cubes. And a 428 will work with headers, by the way. A big block in a 69 bay would definately not be fun to work on, but it would be great to drive. I'm relatively new here, so I don't know if this is a big no no, but over at
www.mustangsandmore.com there are tons of classics guys that would love to help you out with whatever you decided to do.
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Neal
69 stang
351C/4sp