The subframe of the '67-'69 Firebird is basically the same as the '73-74 Olds Omega, which, I believe, was available with an Olds 350 engine. The motor mount position on the Olds 350 is at the same place as on an Olds 455, so you may be able to use the mount system from an Omega with an Olds 350 to mount the Olds 455 in the Firebird subframe.
The '69 Firebird used a metal frame bracket with a rubber & metal mount sandwiched between it and the engine. On the Omega, a solid metal bracket was attached to the engine, and a rubber & metal "clamshell" mount went between the frame & that bracket. The bolt holes in your frame might not be in the right position, but you can drill your own if you need to.
The 455 has a taller deck than the 350, so accessory brackets from the 350 probably won't fit the 455, and you might run into some interference since the 455 is a little wider. It might be easier to just sell the Olds 455 or see if you can find someone with a Pontiac 455 that's willing to swap.