The Mangusta uses the same engine as the 68 Mustang 302 4v, J code engine.
Look here, mustangtek.com, for information on the casting identification numbers.
The number you gave on the heads is only part of the identification number for them. On the top of the head you should also see 4v and the casting date code.
The block could be original but I would expect it to be a C80E casting number for a J code. I would think the C5 is a 289 block and was a replacement. You need to look at the date code for the block also.
The date should be close to the build date of the car. In a Mustang it would be about two weeks earlier. I don't know with a Mangusta but I would think only a couple of months earlier.
Certainly the casting date on the heads should be close to that of the block.
The dates are on the other components like the distributor too. Also the thermostat housing, the water pump, the timing chain cover.