KA alt Core to RB alt

[legit240sx]

so I bought a RB20det to put into my 91 240sx. When i got it in it the rb20 alternator wasn't giving enough juice to keep the car running. we tested it and it was the alt. so I'm at the point where i don't have the money for a quest alt and heard that you could switch the cores. I was wondering if any one has done this and what would be the best way about going about it. thanks for your time,JosiaH