How much time should be allowed for the difference between the trim tag date and the engine casting date? I am looking at a 70 L78 Nova and I have the engine build date but am waiting for a pic of the trim tag.

Generally a few days to a month is the accepted amount of time but is could be up to 6 months or more.
James

James is right, anywhere from a couple of weeks to a few months. It's not what the date is anymore, but what the stampings look like. If you could get some good pics of the deck of the block that will tell you more than the numbers will.