will this wheel fit on the rear of 66 dart convertible, 8 3/4 and what tire to use? thanks in advance.

I don't know for sure, but I'd guess big.... no. Not enough backspace, too much width. Proper sized tire for the width will be way too close to the fender.
I'd look for a 15x6 or 15x7, with at least 4" backspace.
And I'd probably try a 235-60x15 with a seven, or a 225 with a six.