OT: welding thick stainless 304

Anyone have experience welding thick stainless?
I have a bunch of 4x6x3/8 tube i want to weld 1/2 base plate onto . Where do i start?

Depends greatly on what process of welding you have available to you and which you are most comfortable using. They make stainless rods for stick welding. Some of them can be a bear to use. Lots of Flux entrapment. I would bevel the tubing edge as needed. If using the stainless sticks I would have a good preheat on the tubing and the base.

Depending on what welding experience and equipment you have, it should be pretty straight forward. My first choice would be gas shielded flux core MIG welding. In my experience, the flux cored wire is usually shielded with straight CO2. Probably use a bit longer stickout than what you're used to with regular hard wire mild steel mig welding. Use a push welding technique rather than a pulling method. Most likely you'll have to run it flat, no out of position welding. 045 or larger flux core should be available through a weld supply outlet. Do some test runs before moving to your actual joints to see how it runs.

Depending on the filet depth called out or needed for your joint, stick welding with 3/32 or 1/8" rod would be the next logical choice. Running the root with the smaller rod then capping with the larger rod would be my suggestion.
Stainless is going to draw like a bugger so you will see those baseplates curl up toward your weld. The bigger the fillet, the bigger the curl. I'm not sure you'll need the bevel in this case, sounds like a basic filet weld and you may have less chance of slag entrapment without the bevel.

I only wire weld stainless no flux. That thick of material preheating will help a smaller machine weld it but my 350P I turn the start up current up so I'm burning in as soon as I pull the trigger. I never weld thick stainless, just the 7/8in and 1in tubing on boat bow rails. I should say Big Boat bow rails and those I TIG.