I have not had one in a while and I am thinking about getting back on the wagon,I am all ears.
The beauty of the Marlin 336 is that you mostly take it out of the box and use it. The only upgrade I would do is to change the sights. Either add a scope or a good peep sight, and if your eyes are good I'd recommend a peep over a scope by a good margin so as not to mess up the handling.
Just MHO, but a lever action is not a long range bolt gun and doesn't benefit all that much from treating it like one. It's a great handling, reliable hunting rifle. You can do the trigger, you can (as I've done so often) fiddle with the barrel bands and such to try to accurize it. That's fun if you're into such, but 336's generally don't need it.
What caliber were you thinking of getting?
A trigger job is nice, as are new sights like Jim mentioned. Other than that I'd get rid of the factory finish (nothing like a good oil finish on a Marlin, I've got one to do this winter myself) and maybe put a rubber butt pad on it. I do that to mine so they don't slip when leaned up against something.
Then shoot, shoot, and shoot some more. Have someone take a pic of your cheshire cat smile. That's the best kind of upgrade you'll get with a Marlin 30-30!!! or 35 for that matter....
Nikon Prostaff 2-7X32 in Burris Zee Rings, 1" leather sling, hammer extension.
With any Marlin lever, I would remove the white line spacers that they use on the butt plate and pistol grip (if so equipped). I think they look much better without the white.
Take a look at the .308 Marlin express. I love mine. It is a long range lever gun and accurate. Not to mention fun to shoot.
[336,what would be my 1st 3 upgrades?]
1) A Williams or Lyman receiver peep sight
2) A brighter front sight, preferably a fiber-optic bead - like a Williams FireSight.
3) A supply of Hornady LeverEvolution ammunition
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Good advice. There are a number of aftermarket add-ons available but 99% of them are far more beneficial to the supplier than the rifle.
I bought the XS peep sight kit for mine and I love it.
My first Marlin lever was a new Guide Gun in .45-70. Hooked me. I have since picked up 336's in a .35 carbine (full mag) and a .35 rifle (1/2 mag), both from the 50's. I like the "one ragged hole" peep rear sight blade changeout. Some like it, some don't. My favs are also the 94 Win's in .32Win and the Sav 99's.
I have a 1981 336T in 30-30. I did the following:
> Williams peep (side mount)
> XS front blade (the taller one)
> removed the mag tube sling mount and replaced the stock rear band with the newer style band with the sling mount
I'm thinking about slimming the forestock down or looking for a thinner one to replace it.
On my 336SS, I converted it from pistol grip to straight grip, installed a WWG trigger, a XS full rail, and a WWG large loop lever. I have been using it scoped, but I am going to buy the XS peep sight set that fits on the scout rail.
I highly recommend the WWG trigger and large loop lever, but I wouldn't do the full rail again.
DRC big loop.
Brighter front sight.
Oil finish without the white spacers.
My 336's have benefited greatly from the additions of an after-market Rifle Basix trigger and scopes. Since I turned 50, the iron sights are not much use to me. A third upgrade that I have never done, but considered, would be to have the wood trimmed upped some. As they come out of the box, the feel a little like a baseball bat now!