how do i tell the specs of my flexplate

My book doesn't have info on pre 70 351 for some reason.

I also can't find the number you've given in my book. Your flywheel in a 289 with manual trans from a pre-69 mustang should be a casting number like one of the following:
C6ZE-A or -B or C7ZE-B (pn C3AZ-J) that's a 157 tooth gear and 18 5/16" bolts on a 10 1/2" bolt circle.

Flexplate for auto trans: casting C8OP-C or -D (pn C3AZ-L)157 tooth gear with 4 7/16" converter attaching bolts on a 10 1/2" bolt circle.

High performance flywheel: casting C7ZE-A (pn C3OZ-E) same info as above

HP flexplate: casting C7ZP-A (pn C3OZ-E) same info as above


I started to post this yesterday, but without the 351 info I'm not sure if it's much good. Since no one else has replied to your post, I'd recommend you call a Mustang parts vendor and ask them what part you need for the 351 and what the specs on it are. Then you can compare that requirement to the part you already have to determine whether it will work.

I've also seen flywheel specs on the web, but couldn't find my link. One of the toploader transmission builders publishes the specs for flywheels and flexplates on their website. That might be another way to find out without calling someone.

Good luck, I hope this helps a little anyway.