Glen,
This is the procedure I would recommend for welding your K-350. TIG (Tungsten inert gas) for all the "root passes" and FCAW (Flux core) with a cover gas for all the fill out and cover passes.Use filler wire or rods that will produce 70K tensile strength welds. Prepare all your joints with 60 degree included bevels, then weld them "open butt" with the tig process. The cover gas from the tig torch is all the back up shielding you will need while welding the carbon steel parts.No need for ceramics or a gas shield box. You will be back grinding all the root passes anyway you can remove any defect areas then. Once you have a solid root pass in place then move on to FCAW to finish . This brings us to an important point in the discussion. Is your welding equipment adequate to utilize the FCAW process ? The reason I mention this is because most single phase power sources just do not have the electrical "horse power" ( current dens! ity or duty cycle ) to produce acceptable weld integrity. Single phase low amperage machines are fine for sheet metal but not for the steel thicknesses you will be working with in your project. Bottom line, you will need at least a 350 amp CV (constant voltage) source to "push" the .045" Flux core wire.
"Kobelco" makes excellent flux core wire ( .045 " DWA-50 ) and is 70K tensile strength rated. "ESAB" brand is also very good , ( 7100ULTRA in .045") use a 75% argon 25%carbon dioxide for a cover gas.
This is just the tip of the iceberg let me know if you have any questions.
Daniel Lance