Neat old R/Blocks.
If it's in 43 Spanish caliber and chambers rounds made from 348WCF brass,,either rear end of the chamber is very much worn oversize or the chamber has felt the cutting action however slight of a .348 chambering reamer.
The latter action to just slightly enlarge the head portion of the chamber so that it will in fact accept cases made by simply necking up 348 brass.

The 348 case is too large in dia at the head/base to normally chamber in the 43Spanish. Trying to size them down in a set of normal reloading dies will accomplish the decrease right up to the solid base.
The usual route then is to lathe cut the outside of the base the few .000 extra diameter so they will fit.
Trying to use the reloading die to swage the solid case head down will usually crack the large dia mouth die, even if your press could do the job.

The case is also shorter than a 43Spanish when done.
**Ivan the Butcher mentioned these points in a post above.

The best way to go is use the 43Spanish Basic Case and form them in your reloading dies.

If you do want to use 348's, I'd go one step further and use 50-110WCF brass (STarline).
The 348 is based on the 50-110WCF.

You still have to do the outside base trimming as with the 348WCF unless your chamber will accept the unaltered base of the 348case.
But the OAL will be correct with the longer parent case.

LEE made or still makes 43 Spanish dies. I've never used them in that caliber. I do know some of the early production had the shell holder that was cut too 'thin' to accept the standard thickness rim of the 43Sp cartridge. It instead was cut for the very slightly thinner rim of the 348WCF.

Nice lead launchers!