Welds like this are done by the thousands every day.
Verify the code you are working to and comply with preheat(~350F), PWHT (probably none)requirements. Note the exceptions if there are any.
B3 fillers will provide better creep at ~600F than carbon fillers. Carbon fillers will leave your carbon depleted zone in the weld metal and not the carbon HAZ as with B3's, but the carbon weld metal will still be stronger than the carbon base metal.
B2's are acceptable as well, and won't cause carbon migration as much a B3's. Though I'm not sure it matters much.