Find out if your car's door skin, door depth AND plastic door trim/grill can accept the size differance (i'd start at your car specific forum) and if all is good simply cut out yout door's skin to make it work with 6.75''.
My car works with 6.5'' but apparantly not with 6.75'' which is what SR6500 really is.
Here's size differance of 5250 and 6500
and here's my door's speaker opening and sr6500 mounting bracket, you can see that openings is a bit less then needed.
I've marked what i need to cut off with red marker and dremeled it all out
and here's it is nice and neat: