tig welding 400 series stainless to 300 series

[markct]

i have a bunch of 400 series stainless around from some commercial freezers that i stripped the outsides off of, i want to make a maple syrup pan and a few other things from them. i did not know they were 400 series at first, and when making a roofjack for the stovepipe to pass thru i welded some 300 series to some 400 series with 309 filler rod, it seemed to work great, and they arnt rusting, been on the roof a while now. is this ok, is there a better rod for welding 400 to 400? or is 309 ok, anything to do or not to do? or to watch out for? thanks guys