Considering this appears to be for lower temperature service based on materials of construction, I would recommend using the GTAW process or SMAW process with Ni-containing low alloy filler metal, like
SFA 5.28 ER80S- NI2 GTAW process
SFA 5.5 E7018-CIL SMAW process
if your MDMT is below -30 deg F.
If not, ER70S-6 would work for GTAW.