Rear Speaker Measurements

Rear speaker in mount removed from door. Distance between bottom of mounting flange to top of speaker frame. Just under 1.5".

 Measure from bottom of mounting flange to furthest point of speaker. Looks like 2".

So you could bring your speakers forward about about an inch without any worry, probably as much as 1 3/8". The spacers I had made for me were 3/4".
So with my JBL P662s, and the 3/4" spacer, I have about 1/4" more clearance on the window side and 1/8" more clearance on the door panel side.