i have a 98 pintiac firebird i'm thinking about taking the 3.8 v-6 out and putting the 5.7 v-8 in it whats required besides the wiring harness and ecu  

I'm sure the fuel system is different. The K-member is diffferent. The brackets need to be changed for accesarries. If your not real electric and mechanical savy, I would have a place do it for you. Shops like Speed inc. in Schaumburg,IL has done it.
http://www.ls1speed.com/
They can tell you.