There should be no problem using the RB flange.   Weld the pipes to the
flange  from the  inside as per the RB instructions and use 309 filler rod.

Perry Casson

Thanks Perry.  BTW, where are these RB welding instructions?  I just looked through the catalog, and also the web page trying to find any mention of this.  All I found were pictures of RB headers that were welded on the outside. 

Thanks,

Rusty