Jon,
If I may, I don't think your problem would be the Charpy's with FCAW. ESAB has an excellent 71T-1 that routinely hits better than 20 @ -50. I used it exclusively in my nuky quals intended for SA-333 Gr 6 (also tested at -50). The weld metal never failed. What I would beware of is the HAZ/UBM comparison with SA-516-70. Your 516-70 may stop a hammer at 0deg. Esepecially on a 180 Ft/lb machine. See if you can find a 240. 516 is terrific stuff, with fine grains. And with hammer stoppers you have no nuke qual. Unless your qualing to the new Edition Section III.
The old Section III's are merciless, based upon old steel making technology. I mean, 40 years ago nobody much saw hammer stoppers at low temp. Not on 240's.
I had to drive the temps down to get the specimen to even break so that I had data. And even then I was hittin mle's in the low 90's, AND FAILING, because I couldn't beat the BM. The typical was like 90, 92, 89 for the HAZ and 92, 93, 91 for the BM. Thats a failure. A FAILURE!! Under the old Editions.