Wrong area, but I'll answer because I'm a softie for audio.

I have 6.5's up front and 6x9's in back and I LOVE the improvement in the range and depth over the stock 5.25's / 6.75's

You will have to modify or purchase new mounts to get the bigger speakers to fit. GS and up cars had speaker pods for the front doors that will mount 6.5" speakers quite easily, but they will NOT fit behind manual door panels. You will either need a GS or to swap over to power doors (well worth the time)

For the rears, 90% of the cars out there came with an oversized 6.5" (really a 6.75" ). If you can find the speaker mounts from a car that had an infinity stereo they had 6x9" speakers in the back, but last I checked Mitsu doesn't have a valid part # for the mount x( look for a used set or do like I did and just cut them out to fit the 6x9