Take a look at the 30-30 Winchester, the 0.422 case diameter can be fireformed to the 0.429 diameter of the 401 WSL or you could expand it if you can find the proper tools at RCBS or Dillon. Length is plenty long enough to trim back to the 1.50 inch case length of the 401 WSL. The next aspect is that you'll need to cut down the larger rim diameter of the 30-30 to that of the 401, something that would be easiest to do in a lathe but in a pinch you could do it in a 1/2 inch drill with some sandpaper. The final aspect is the rim thickness, the 30-30 has a 0.060 inch rim thickness and the 401 WSL is listed as 0.050 in Wikipedia. this is a case where you may want to make up one sample and see if it will fit under the extractor on the Winchester, if so you are good to go with the 30-30 rim thickness. If not, it would be best to find a machine shop that would be willing to face off a batch or your cases for a moderate charge. The other option will be to sand the rear face down but that will take developing the skill to keep the rear face of the case flat while you sand it down.

Now, one note about fireforming. The 0.007 inch change in size isn't a huge amount and I don't think you'll see any cases split. However it could happen in one or two cases and the most likely consequence will be a case stuck in the chamber. I also expect that you'll see an area near the base that won't expand fully due to thickening of the case walls near the base. That I wouldn't worry about, the only way to address that is with an expand punch.

Other than that the 30-30 operates at a slightly higher pressure than the 401 WSL so a case head separation is highly unlikely as long as you don't try and re-use your fabricated cases too many times. Hint, if you see a depressed band about 5/16 inch above the base after firing that case has reached the point of failure due to stretch.

Now, for cutting the 30-30 down. Trying to do this with a case trimmer would take forever, just look at the difference in length. What I would suggest is to chuck the case in a 1/2 inch drill while spinning and then use a Dremel with a diamond cut off wheel to cut the case just a touch long while spinning the case in the case trimmer. Naturally, if you have a small lathe that would be an ideal solution. You may also need some bees wax to lubricate the cut off wheel because brass tends to stick to almost any grinding media. Time to cut one case, at a guess plan on about a minute per case. After that trimming to length should be relatively quick provided your cuts are fairly close. I'd set the goal for the as cut length at about 1/16 inch too long but if you can figure how to position the Dremel accurately enough you may be able to cut them closer.