Normal 6.5's will fit with MDF adapters.

The only cutting you need to do is the thin plastic bit that creates a duct to the stock speaker.

This is completely non structural, and not visible when the door card is mounted.

More extensive modification may be required for oversized

(6.75") speakers, or speakers with a particularly large mounting depth.

For best results, I'd recommend getting a small amp to power them.

Not much point getting nice component speakers and then running them off the crap head unit built in amp!