I have 2 Marlin 336's,one is an A,other a carbine.Similar vintage late 70's.The A has some severe pitting on receiver,the carbine is decent.I'd like to change the complete front half's in the 2 guns.How difficult is it to change the barrels or should I take it to a smith.I did that once already,he had it a year,never touched it lol

You’ll need at a minimum a barrel vise and an action wrench. Good possibility of your needing a lathe too.

When I changed the barrel on mine, I drilled a hole in a block of wood and sliced it down the middle for the barrel vise. Then using the instructions in Ackley's book, used a large crescent, with cardboard to protect the finish, on the receiver. The new one headspaced correctly with no additional work.
Your mileage may vary.

If the headspace is not correct, you will need to get a new locking block and fit it. Check you tube for instructional videos. Mel

Thank you all,probably best to take it to a smith lol