Well, the Olds 403 was found in these cars, and for sure headers were
available for that.
I don't know enough about Oldsmobile engines to know how similar the
exhaust port layout is on 403 vs. 455, but there are some comments
here that claim that 403 headers will more or less work:
http://www.youtube.com/watch?v=449TI3OMUJc
They shouldn't fit, given that Jegs has different part numbers for 403
headers (all of which fit Cutlass and Firebird) and 455 headers (which
only fit Cutlass).
Talking with a header company is your best bet.
....Ed