I'm no help but I do have a 4" blued made in 1978. I basically purchased it as n investment as I got a really good price on it. If you really want a 6 then I'd hold out for one - you'll run across one at some point. I guess I'm having a hard time figuring out why the guy is sticking to his guns on the year he is telling you? If I remember correctly, the serial # are listed on proof house - an easy thing for him to look up. The bar code on the box alone should be a big clue that it isn't 70s.
You don't say what he is asking for it . . . if he is adamant about the date, he probably is at the top of the value. But, if it were reasonable and you had the extra $ to put in to it, yiou might dicker and get it and use it along the way for trading stock?
Personally, I don't really care to shoot mine that much - I have other pistols I much prefer. But, to me, a Python is an investment that will just increase in value. I also don't like shiny ones - but everybody has their likes and dislikes. If it could be bought right - I really don't think you'd have a problem finding a buyer - the plus is it has the box - and maybe you could make a little profit to further finance the 6" you want.