Will the BOSE front door speakers fit in my Bonneville With the standard 5.25" front door speakers? I'm thinking I want to upgrade the base Delco System to the Delco/Bose system, which should have better, clearer sound, right?

The sound difference would be Marginal at best. Any cheap aftermarket speaker installed properly should sound better then an upgraded OE setup. Some Sound deadening behind door speakers goes A LONG WAY, and makes the mids much more present.

Do note that the Corvette Bose speakers have amps attached to them. They will not give the best sound without those amps hooked up.

I wouldn't suggest corvette bose speakers. Even if you get the C5 bose, it wouldn't be all that great.