Have you looked into Graflex twin cutter boring heads. Seco Carbaloy bought them out. We have excellent results with these on ASTM 536 Grade 50 which is much harder than A36. With the right grade insert stainless should be no problem. These also have optional insert holders that are offset about .010" in Z so you can use one to rough and the other to finish in the same pass.
Only drawback is the $$$$$$.