You need to get the B&M flexplate made specifically for this purpose. Using an older, smaller flexplate will not work, as you already know. The B&M is made for the post-'82 5.0L's and C4 autos. Can't remember the exact part number, but you should be able to find it online. I did. Think the URL is
http://www.bmracing.com
BTW, the starter should work OK. I had to fudge around with starters for a while to get one that would work with my headers.

I'm pretty sure almost all small block Ford starters have the same gear tooth count on them. At least the bendix for them is all the same one!