Grade 409 / 1.4512 titanium stabilised ferritic stainless steel is often referred to as 'muffler grade stainless steel' since the major bulk usage of this material is for automotive exhaust systems.
Grade 409 / 1.4512 is a rolled product (sheet, coil and strip) material usually available up to a maximum of 2.5mm thickness.
Grade 1.4003 is similar though not a direct equivalent but is available in thicker sheets and plates.
Weldability of grade 409 is good up to about 2.5mm thick.
If welding of thicker material is necessary, then grade 1.4003 should be selected.