3914678 was used on all small blocks in 1968 from the 327/210, to the 350/295 to the 302 MO blocks.
HH doesn't match up with anything in 68 though. Could you be missreading the block code?
In 1968 the follow codes were also used.
HB - 327/275 M/T
HI - 327/250 M/T
HK - 327/250 PG
HL - 327/250 M/T HD Clutch
HN - 327/? PG
Interesting enough, I couldn't find any 350's beginning or ending in "H" in 1968. (I just did a quick browse though.)
'67 RS Coupe - Gold on Gold, 327/210, PG and mostly original
'67 Plain Jane Coupe - Penske Blue, Z/28 stripes, 350HO & Tremec TKO500
'95 Z28 Coupe - Dead stock (seriously)