[Bill Pritchard]

You're right, the first position should be either a letter 'F' or 'T'. It's not too difficult to imagine that someone saw either of those letters as a number '1'. In 65, HC would be a 250hp 327 from a 65 full size car....but HG would be a 375hp fuelie 327 from a Corvette. And from what I know, there should be a CON VIN stamp on that pad if it was originally a Corvette engine. If it really is an HC engine, that means it came from a full size car and probably would NOT have had the CON VIN stamp.

Those pad stampings can be pretty tricky to read, especially if there's been a significant amount of surface rust present over many years. You either need to get someone who really knows their stampings to look at it for you, look at it yourself, or get some really good quality pictures.