Who do y'all recommend for taking a stock 336 w 20" barrel and shortening mag tube to 2/3 length. Would need smith to furnish parts too.
Approx cost?

Check "gunpartscorp.com"...they might have that/those parts in stock. You have to do some research as the mounting points vary between barrel lengths and tube lengths.

I did this to one of my 336's many years ago. You also have to be sure the tube is for the case size, i.e. 30-30 vs 45-70.

It is just a simple tube size correction, maybe a mill job for the dovetail mount and maybe a mill for the through bolt on the barrel/tube connector. I found the correct parts and didn't have to deal, but I was going from a 30-30 to a 444 M and all the parts were available for the 22" 444 M barrel.

ANY levergun gunsmith can do it or if you want it permanent can just cut the OEM tube down to size and remount it.

It DOES make it look kooler.

Good Hunting

Ok, it is not that easy. Assuming your 336 has the barrel band at the muzzle, that is what holds the tube in. Shortening it will leave nothing to hold the tube. Might change the forearm to a rifle type and use a band dovetailed into the barrel which holds the tube with a cross pin.
Or use a dovetail piece to hold the screw.
Or move the existing barrel band back, which would work but look funny.
Could make the screw that holds the tube cap in go into the barrel like Winchesters do.
I've shortened many of them. Lots of ways to do it.
Cost depends on what rifle/carbine you have and what you want configuration you want when done.

You will have to have a 'hanger' cut in the bottom of the barrel to attach the shortened tube. Then, you will have the slot for a hanger at the top possibly. If you have a top barrel band, then not.
If it is a common chambering like 30-30 or 35 Remington.... the easy solution is to sell yours and buy the variation you want. It will not be very hard to find with millions having been made.

Dennis Olson, Plains, Mt. 406-826-3790