Yes, HF was the two letter frame ID code for the '70 455 hardtop frame.
'71 Pontiac 455 A-bodys (non converts) used one of two different 455 frame codes. '72's with factory 455 reverted back to one 455 specific code. Over in the Buick camp, '70-72 GS 455's also received their own specfic code frames.
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