340 engine

I do not totally ungerstand what numbers matching means. I am looking at a 68, 318 fastback with a 67 front end and grill with a 340 automatic installed. I am really concerned about the pedigree of the car. The title matches the VIN on the dash (questional originality) as a 68 but the number on the radiator core support is different. I'm told that there is another VIN in the trunk...??? Please help!!