Flex plate replacement Two Questions

Hi,
I posted questions about this in another tread and learned a lot. But I still have a couple of questions.
First the history of my problem:
A few years ago I had the flexplate replaced on my 85 F150,351 Automatic.The original plate had missing teeth and it was burning up the starters. I asked my mechanic if they made flex plates out of hardened steel and he said they did. So I had him put one of those in, hoping I would never have to deal with that problem again.
After the new plate was installed the starter was really noisy.. Something was not right.The truck sat for a few years but now that I'm driving it again that noise got worse and I think I have missing teeth for the second time.
Someone mentioned on this forum that the flex plate is supposed to be beveled. I'm not sure I know what that means. Are the teeth beveled in an angle where the starter gear engages? Mine are square, no bevel. Did I get the wrong flex plate installed? Is that why my starter grinds like crazy and eventually fails?
Question One:
Bevel or no bevel what is the truck supposed to have?

The second question :
It was recommended to me I have the mechanic install a PMGR stater from a 1990 Lincoln Towncar or newer.
At Napa they told me that these starters come with 10 teeth and also with 9 teeth.  Which one should I get?
Thank you