In my opinion, any 34 year old bike that turns up complete and unmolested is worth a look. Especially at that price.
I've owned both 250 and 350 '76 models and enjoyed them for what they were. I especially enjoyed Sunday mornings following dirt roads through the hills on the 350.
Yes a '74 has more value and seems to have more appeal as a racer, but I'd grab it if I were you.
I'd prefer to see it kept in original condition, but if you want to do the Vinduro thing on it, read up on Bill Bell's Baja bikes. Lots of money to be spent and fun to be had on one of those.
btw I've got plenty of articles on them if you're interested.
If you don't want it, I do!