I think on an 03 Reg cab the seat belt bolt has to come out and the panel should pop off. But don't quote me on that.
Rear speakers are 5.25 inches. Get the speaker harness adapters so they're plug and play. Unscrew the old speakers, plug the new ones in and screw them in. May need to use self tappers if the holes don't line up. Polk audio makes nice midrange stuff but nothing is going to make a huge difference with the stock head unit. Especially if the rears are the only ones getting replaced