Offshoreonly.com - 79 454 series 400

I would like information on the Mercrusier Series 400. I have a 79 32 Excalibur with 454 series 400's the casting in the blocks and heads are 78. I have the plates off the trans and the plate on the valve covers are matching numbers so I am thinking that these are the original engines. When I was going through them I found out that they have been rebuilt. The serial numbers are 5346873S and 5346876S. I would like to know how a 79 boat gets 78 casting and a model that didn't come out until 80. I have went to my local Mercruiser dealer to get parts and when we put the engine serial numbers they don't exist. If anyone can shed and light on this subject it would be greatly appreciated.

Thanks,
David