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

unless there's a whole bunch more room on a 66 vs a 65 and you're high enough in the rear to fully clear the wheel well lip, no.

i'm running 15x7 w/ 4.25 and it's *tight* (i am, however very low).

stock mopar rims in 7" generally came with 4.25 so you'd need more to bump 8