I did this same thing with these....

Pyle PLPW6D Dual Voice Coil Subwoofer - Walmart.com

some shaving of the back of the box and I drilled some holes to match up with the screw holes on the pyle and put plastic wall anchors behind them to secure the screws behind the box.

I'm wondering what removing the capacitors in the door speakers will result in at this time with the new 4 ohm subs.

Anyone have some insight? I think I am going to do the door speakers next, then the head unit, then the amp.