I have searched the forums and found that i can replace the 4in. speakers with 6.5in. speakers. But, a couple say that there is no way that they will fit in the rear. I want to replace all my 4in. speakers in my sc 400 with infinities.

 

From what I understand, it is possible but will require a good bit of cutting metal in the rear deck and needing a bigger enclosure up front. The 5.25" speakers are easier to fit for obvious reasons...these are what I decided to go with.
Maybe someone who has actually replaced the stock speakers with 6.5" can provide more info about how much work it will take.

 

I installed a 6.5" Infinity Kappa 3-way in the stock rear hole and all I had to do was drill 2 extra mounting holes, one from the rear seat and one from inside the trunk. It definitely fits.

If you search, you'll find plenty of posts on CL showing 5.25" and 6.5" speakers instralled in the front position.