To be honest, I have never even heard of a 73Cr/24Ni alloy, to my memory. You sure that isn't reversed? If it were to have a name I might suggest crackalloy.
As for fillers, if this is indeed the alloy, your choices are either a matching chemistry rod wherein the alloy manufacturer produces welding material as well becasue nobody else has taken it on yet (since there isn't any standard fillers for this under AWS that I know of), or some nickel base alloy such as NiCr-3, etc., which is almost the reverse.
Wonder where this would fall in a Scheaffler. If it would even fall in Scheaffler or a WRC. I think 73 Cr is beyond their scope. And even given the synergic relationship of Cr (understanding that Cr is a ferrite stablizer) in establishing austenite, would this alloy be autenitic or ferritic. I'm thinkin ferritic.

Thank to you for assistance in this matter.

Mentioned Material is use in welding to Fired Heater of Aromatics Project.
However, I'm wating for the Method Statement and Material spec. from THCHNIP.
Once I get that, I would like to inform you all.