jaf2021, many times, the GT rating and traditional (SS/HA, IA, etc) rating are different.
1967 Chevy 327/275, in GT, is rated:
295, OEM head, stick
299, OEM head, auto
300, replacement head, stick
304, replacement head, auto
So be sure to look out for this info when choosing an engine for GT purposes.
As far as an engine for GT, pretty much any motor is fair game, if it is corporately-correct (any Chevy engine in a Chevy, Buick, Olds, Pontiac, Cadilac, etc). Just be sure to use the GT rating when figuring legal Class(es) and weights. Youcould put a 2012 COPO motor in, if you wanted to.
Hope this answers your question.