what year models will swap over to a 1999 firebird formula

I would say starting with model year 1998 through 2002 all those parts are identical. I believe the cars were engineered for a V8 but then they gave the customer an option to have a V6 installed so it's the same engine bay so I would say you could swap the engines in them all day long! You may even be able to use a 1993 - 1997 -Firebird but they revised " made improvements" on the Generation 4 model in 1998 so not sure about the 1993-1997 but I'm pretty sure 98 through 2002 you can do this but check with the dealer too.