Dave, I too have similar interests although I am not a sound engineer, SQ is all that matters to me. Firstly, you will not get any serious bass out of the front. The best thing to do is use an amp with a high pass filter at say 100-120hz to prevent any bass going to them at all. This will allow those speakers to perform best at the frequencies they are designed for. As you know the majority of imaging and soundstage is produced by the tweeter. A good set of components will have a comprehensive crossover which should ensure a smooth transition to the high frequencies. The position of the tweeters is the critical thing. I use double sided tape to hold my tweeters in place so that they can easily be moved in order to find the best position. I often find that bouncing the sound off the windscreen can produce the best image but if the tweeter is too clost it can take on a sort of horn loading characteristic. Trial and error is the only way though.
As I said above forget the rear speakers and install a sub crossed over below 100hz. This will fill in the low end. This will make it a three way system with clarity and detail at the top end and real punch at the bottom, a bit along the lines of a JBL 4311 or something.
As for the 3" hole, can you not just leave the old speaker in it but disconnect it?