HOwdy Cam
Congratulations on your new rifle! I can say from experience that Howard is a Great Guy. He was kind enough to loan us his Short Wolfe reamer over a decade ago so we could quickly begin our research on big 30 cals. I was in awe of his unselfishness and willingness to help people.
We got away from the 30-378 for a couple of reasons that I will not go into, but it is a fine cartridge, and if I remember H50 BMG was the most consistent performer across a range of conditions (temps) and did well on velocity with 200 grainers and heavier.
We had to meet certain standards for velocity (and pressure variances) that you are not subject to, and we noted in our research that in cold weather, there were some ball powders, that produced amazing velocity and accuracy, but had problems when things got above 45 degrees F.
I would defer to DC on the powder and load info on this case, because our data is old and there are guys that have shot a lot of this recently, with newer lots of powder.
Would really like to know how it shoots, and how Howard is doing. Good luck, but I bet you won't need it!