I've helped a few people install speakers bigger than meant to go in a spot in the last few years and went through a pretty good learning experience making a system loud & clear in a transit van. Speakers sticking out a little sound better than the way many factory speakers are a little recessed behind the panel. One negative thing about the tweeters being down there is they won't sound as good from your seating position as if they were higher up. If you were capable of making angled spacers, that would help. Also hope you have some sort of decent amp to power 2 aftermarket speakers per door, guessing you don't. How you wire 2 speakers per channel makes a difference also. Wiring parallel will make the amp put out more, but at a 2ohm load if using 4 ohm speakers will make the amp clip if not 2 ohm stable. This means you need to wire in series, which will make an 8 ohm load to the amp.
If I'm giving your mind a headache, that's ok. Building a truly good stereo is kinda like building a really good hot rod. If you have a little talent, you can start out not knowing much and constantly upgrade both your knowledge & equipment from there. At 52 years old now, I have the best sounding stereo I ever did in a work van of all places. How you build enclosures, wire things, sound deaden the vehicle, etc, can be more important than just spending money on more expensive components.
