I start with if you are real cheap and have a lot of time then reforming 348 win brass into 43 spanish is the way to go. But if you think this is easy Try one case and you will say this is a lot of work. You can not just run a 348 case in a 43 spanish die and come out a 43 case. The base dia is too large. It will take a real good press to squeese the base down. Most use a lathe to turn down the bases. Now the rim dia is to small for a worn extractor, so the extractor will have to be built up or use a rod the get the cases out. Allso the rim thicknes is to small on the 348 but that not a real problem with a shoulder cart . Just size the cases to head space on the sholder. One you have done all that and you get it to chamber you now have to fireform the case to chamber fit and then trim to size or use an expander to open up the neck and then finisn fire forming. Now that you have buffalo arms that has reformed 348 brass for the 43spanish rem it is a less hassel to some of us to just buy the brass,bullits,load and shoot.
also check your chambers with a cast. Back in the 70d,s some gunsmiths opened up the spanish chambers with a 348 reamer to fit the 348 base dia. so 348 brass could be used without swagging the brass base. It was donr when 43 spanish brass was hard to come by and all origenal were berdan primed. There is allso the 43 egyption that is very much like the 43 spanish but has no case that is close to reform with ease. Them mill rifles will have you scratchen your head some time tryen to figure out what they take?