I have to respectfully disagree with both these statements (which is kinda funny since you are simply talking about what you have done to your own car ).
1. 215/50/17 on a 10 inch rim seems like an impossibility to be since the section width on a 215 50 17 is 8.5-8.9 inches wide depending on the manufacture.
2. If you are talking about the 1965 mustang you have as your avatar you are not running a 10 inch wide wheel with 4.25 inch backspacing. I know this because if you were your rim would stick out past the fender of the car 2 inches and in the picture your rim and tire do not stick out.
Now, if you have tubbed the car and shortened the rear then yes you could be running them, but if had done that type of work, I assume you would have mentioned it.
I found some nice bullett deep dish rims on sale for cheap but the sizes are 18' rim and a 9' wide the offset is 24mm would these fit? and would they be right under the lip?
I have a 1967 mustang coupe and these are supose to go on the rear....
I would recommend a .5 inch spacer to adjust the backspacing on .24 mm offset wheels from 6 inches to 5.5 inches. Ideally I think they would fit better with a .75 inch spacer, but I would not recommend going above .5 inch on a slip in spacer and you can't get a bolt on adapter that thin. As some one else mentioned, if you have converter to a disc brake setup in the rear I would look close to make sure you have caliper clearance.