VonGruff, your post prompted me to take a look at my own 303 Lee Speed and my guess is that the .375s would feed just fine.
The ramp is quite steep in my gun but it looks like the carrier will angle the tips of the cartridges up enough that feeding is kept quite smooth.
The rounded nose bullets of the day would most likely feed quite smoothly too from the same magazine but I would be interested in seeing if the follower is any different in the .375 as opposed to the 303. I would think that a follower that pushes the front end of the cartridge up would help in feeding.