The second Gen has Pontiac mounts already on it if it came from a Firebird with a Pontiac V8, if not it's pretty simple to change one over to Pontiac mounts, if it was from a camaro. Really the mounts are the minor if you're swapping a complete stub onto an early trunnion equipped Pontiac.
Not sure what transmission you have behind that 455, but going to the later sub frame would also make using any late style transmission pretty easy.
IMO it's easier to just take all the older technology stuff off and swap it to later model, than trying to find ancient 58 parts and still end up with older technology, but I appreciate cars that handle well, and can corner well........
If you decide to do the G body front clip as outlined above the frame rails are closer together than the F body, making the Pontiac engine a tighter fit.
In the end it's your car, and your choice of which way you decide to go, good luck with whatever you decide.....
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