I am sorry I don't have any chronograph data with 250 grain bullets. When I tried load development it was too cold for my Chrony.
But, I am going to make powder recommendations. Go with faster burning than slow burning. I recommend trying N135 or IMR 3031 first, then try IMR 4895 or H4895 or AA2495. The last three are all about the same burn rate. Don't use ball powders, I had hangfires with AA2520, a ball powder that is supposed to have the same pressure curve as IMR 4895. Stick with stick powders. Don't go any slower than IMR 4064/Varget.
I sent my Dumoulin Mauser action, a 1:14 twist Midway barrel, and stock to my bolt gun smith. It will be awhile before he assembles, beds, then I will go out and test in this new 35 Whelen.
By the way, expect the rifle to kick. A 250 grain bullet at 2500 fps hits hard both ways!