Yes, that is definitely a 400. Build date is "J" = Oct. "25", 197"3". The YH would indicate it was used in '74 (which would agree with the late '73 build date) and was originally a 175/190hp 2bbl engine.
And yes, the 4 digit code on the head is also a date code, read just like the engine date code. "C" = March "24", 197"7".
'77 6X heads have 2.11" intake and 1.66" exhaust valves and 101cc chambers. These are possiblly the original heads for your car and maybe just the short block was replaced.