Must admit I find the 348 and 341 very different - I have Lowndes, Hallam and Northcote on the 348 and wear 9.5E on all of them, it's a good fit for me, but when I tried on two different models on the 341 not only was the 9.5 too big, even the 9 had huge heel slip But many others do say they find the two lasts quite similar. 341 definitely runs the larger of the two, though.
@BaldwinP thanks for the cautionary tale. I will make sure that I mentally prepare for possible disappointment, of having to return my first order of 341 last footwear back to C&J in the UK for sizing down. Also, I will ensure that I request 341 in-socks too.