Have you gotten as close as possible with (repeated) annealing and incremental expander plugs in a die? The less you have to rely on fireforming with such things as Cream-of-Wheat and the like the better off you are. At least in my experience, I've had decidedly mixed results with blanks made from pistol powder and meal.

Another approach, but it's pretty messy, is to make up or buy a hydraulic case expander die. I used one from Hornady to form .22 K-Hornet brass without fireforming.

I've reformed 348 cases when i've had to, but it's a not exact process, and you have some losses from cases that don't form correctly.  And 348 is fairly expensive to start with.

Why don't you just start with correct 43 Spanish?  It's as easy to get as 348 is.

When I fireform, I anneal the portion to be formed, and use a small charge of medium fast powder covered over with grits or cream of wheat or similar coarse milled grain.

I use the Lyman M dies, but I put the plugs into my drill press chuck and simply turn the step off with a file while spinning the plugs. I also buy .38 and .45 caliber plugs as spares to turn down to the exact size I need to expand the necks. I bought my M dies in Short and Long used at our OAC gun show and some clever machinist milled the sides open 180 degrees apart so you can watch the plug enter the case mouth and expand. A genius alteration that both Lyman and RCBS should do at the factory!

I'm not sure the .348 rims are thick enough to make .43 Spanish? I believe BACO .348 brass has the rims swaged to offset the edge and create a thicker rim for proper headspace.

He's got it, alright. Got some coming. Thanks.