How has the Marlin 336 changes in 35 years?

"Russ" gave you the only functional difference that I'm aware of(the cross bolt safety)

And as the others have said, IMO, the wood was a bit nicer on the older models.

My suggestion though would be, not to over pay just because it's old. The Marlin 336 is not a collectors rifle. Value has to do with condition not age.

Keep in mind that generally this time of the year the Marlin 336 will be on sale and it's not unusual to see a factory mounted scope included with the rifle for $350 or so. Dick's Sporting Goods has been selling one at that price for 2-3 years now and it seems like several others(Bass Pro Shop, etc) had a sale or two in that price range($350-$399)

Marlin 336 - near new condition

So if you can get the rifle for $250-$300 that's probably a pretty good deal. Less than $250 would be an excellent deal and if it's less than $200, I hope you're not stealing from a friend or family(yes, that would be humor)

Unchanged in a mere 35 years; I would probably like the older one as the wood was better back then. I have a 10/22 I bought back in 72 that has a walnut stock much nicer than the wood available today.

The cross bolt safety was added in the mid-80s. It's most important contribution (IMO) was that it makes it safer to move the hammer from full-cock to half cock position and maybe adds a little piece of mind. No really a big deal in the field. 336 users did just fine without the additional safety for almost 50 years and 100+ years for lever guns in general.

The older models have a better quality wood stock.

The gun is basically the same, in fact the older gun would be my preference. I have one I purchased new in 1972 never had any problems.