RicinYakima
High Steppes of Eastern Washington
Having done two 35 Whelen's on Springfield actions, neither required any magazine work if you didn't try to load 38 WC bullets. The 200 grains RCBS seated to the crimp groove will work just fine. My friend wanted a 358 Win and he did have some slight issues with short cast bullets. It seems the short case doesn't rotate up soon enough to get the bullet into the cone breach.Rebarreling for 35 Whelen might require some tweaking of the feed rails as the necks are wider than the original 30-06 caliber chambering that it was originally designed for. I have a .358 Winchester barrelled Mauser 98. It's a great cast bullet cartridge but I had to tweak the Mauser's feed rails for that cartridge's 51mm shorter length and wider neck than the original 8x57 cartridge that the receiver was designed for before it would feed smoothly.
I don't have any experience with a 40 Whelen cartridge but I feel you might have same problem in feeding from the magazine as I did with my .358Win.
Bruce
I solved the same issue with a 7.62X39 conversion by cutting off the follower and using a plug on the back of the magazine well. A Remington 700 short action magazine spring worked perfectly.