Starter & Bellhousing

I have a 1966 Mustang that is a hodge podge of parts. From what I can tell it has a 1974 C4 transmission and an early 80's V8. It is an automatic. The starter grinds on starting, terrible noise !! I have replaced the flex plate which was slightly bent with the same result. I have tried 3 different starters all with the same result. I have also replaced the spacer betweeen the bell housing and engine. I have also tried several solenoids and replaced the ignition switch. Nothing helped. I now wonder if I have the correct bell housing. The car originally had a 200 6 cylinder. I know the transmission was replaced but don't know about the bell housing. How can I tell and where is the casting number? What bell housing should I have for this engine and transmission. Any help appreciated.

Tom