HF translates to any of these......

HF 1965 327 h/p 300 4 Vette
HF 1966 327 Powerglide, A.I.R. 275 4 full size
HF 1969 350 Powerglide 275 2 A F X
HF 1978 350 LM1 170 4 Firebird

It seems to have the later....after 68 heads on it, so I am guessing the last 2....more likely the 78