Do not try to fit something , get something that fits.
Those speakers are in the doors are they not ?
Your best bet is to look on Crutchfield and see what they have that fits, then see what you can find at Walmart or the like that is similar ( Walmart used to carry Pioneer, have no idea anymore)
Of course, you probably already have the 6x9's (old sckool)
fitting them in the door could be an issue with the window. Doors ain't as big as they used to be.
As for the factory head unit, be aware that SOME truck funtions are based out of the radio, On a base model truck, it's mostly door/seatbelt chime and other annoyances ( if it were a higher line model, you'd have actual vehicle support issues, which in my mind is a fawking stupid ass way to build a vehilce, the new ones are even worse)
I've forgotten a lot about the "entertainment" systems on GM trucks, but they can be a PITA
