I suppose D1.3 could be used for your application. D1.3:98 (page 9) 2.3.3 Fillet Welds:
2.3.3.1 Minimum Length
Minimum length shall be 3/4".
2.3.3.2 Leg Sizes:
Leg sizes of lap joints shall be equal to the thickness of the thinner sheet steel(see fig 2.7). The leg size of the T-joint fillet welds shall be equal to the thickness of the thinnest sheet steel(see fig 2.8).
In my brief scan of D1.3, I did not see any info concerning the spacing of the stitch welds, but would venture to guess that would be up to the engineer or designer to achieve the desired joint strength for the application.
John Wright