I have one of the 400 Whelen cartridges loaded at Frankford Arsenal with a FA 22-R headstamp. It has a hollow point bullet which may or may not be a WT&C. Whelen used commercial bullets, and the WT&C would have been the one most available at the time. It measures .410" at the case mouth. If you think it may be a WT&C bullet I could probably pull it to make a better measurement.
There was an earlier thread discussing the Frankford Arsenal cartridges loaded by Whelen. You’d have to find it and see what it said. I don’t remember exactly.
Regarding the comment about the 400 Whelen not having an adequate shoulder for headspacing, the originals made at Frankford Arsenal were made from cylindrical Cal .30 cases. Later cartridges made from necked up 30-06 cases do have a smaller shoulder but they are not the original design by Whelen.