Yes.
I ALWAYS do this.
you can buy rad adaptors that make it a screw in bolt in affair off of Ebay.
just type in 240sx 6.5 speaker and you'll find them.
best $20 iever spent. seriously. I have infinity kappas at all four corners, but will upgrade to diamond audio sometime next year.
the other thing you can do, is get the infinity plates that replace the crappy 4x6 with a nice 4" mid round speaker and a 2" tweeter and a 1" tweeter 3 way component plate set.
Don't forget to run all new speaker wires, and have a good quality ground!
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Advocate for the People's Republic of Awesome
rest in peace tim.