16-8-2 is used commonly as a substitute for 347 for high temp applications, but it is just that-a substitute. There is no reason that I can think of that 347 filler wouldn't be acceptable for the service intended of a 347 base material, except that the Molybdenum in 16-8-2 is considsered to improve the creep rupture ductuility of CrNi SS's. The main problem however with 347, of course, is mircofissuring. 347 is actually quite sensitive.
Keep in mind, if you decide upon 16-8-2, that the ferrite will most likely prove to be quite low, if you test it. Its OK. Even with  low ferrite this stuff is remarkably crack resistant.
But the decision needs to be engineered. How important is the reduced high temp creep ductility of 347? How much restraint is there upon the weld joint-which woul dindicate that perhaps 16-8-2 is the better choice? What are the varying costs of eachof the fillers in your area-And should this be compared with the possbility of repairs should oyu discover microfissures int eh 347? Perhaps a review of service history of the part is in order.