You can get adapters to put 6.5s or 5.25s in the doors. I'd get 6.5s though. I dunno if it's the same on a '98, but on my 2000 the factory amps are build into the speakers, so you'd have to find a way to wire the amp to your new crossovers, or just rewire the truck and run it off the head unit (better option) or get a 4 channel amp. As far as brand, how much do you want to spend? I was never a fan of how MB Quart stuff sounds.