If you use a MIG welder it's a spool of stainless wire.
For TIG welding it's stainless wire rods
For Stick welding it's a flux coated stainless rod.

To get a little more in detail you should know what the bed strip is made of, 304, 409 or whatever alloy. Then your welding supplier can recommend the proper wire or rod for joining it to mild steel.

Same as any normal welding you just buy stainless "filler" instead of the usual plain steel.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-