I mounted 4.5 inch round speakers into the kick panels of my 69 cutlass, which should be almost identical to chevelle.
The first time, I mounted them behind the air vents that exist in the kick panels because I did not want to cut the original panels. But it was alot of work to modify the vent, which was not functional after that.
After I year, I removed them from the air vents and bought another set of original kick panels. I cut a hole for the speakers in the kick panel, in front of the air vents (non air car), and put the speaker into the hole and used the mesh cover that came with the round speaker. It cleared the parking brake pedal, just barely. I like the sound much better from this set. I think I had to cut a bit of metal with my air saw to have the speaker recess enough into the kick panel metal. But not much metal had to be removed at all. There is a big hollow area there. I got my second set of kick panels from a junk yard for 15 bucks, so I didn't go for the moulded custom autosound style. I thought they wouldn't sell them without speakers installed. I think you could easily mount 5 inch round into the original kicks. Not sure about 6 inch, but probably. The back side metal may have to be cut to get the magnet into the hollow, but that shouldn't stop you. Go for the sound, it's wonderful to crank tunes on an old car.