Jon;
I have seen a few Union Arms pumps over the years, but have absolutely no personal experience with them. I do not even know how they bolt up.
he only ones I remember in particular as to being somewhat weak are some models of the early Marlins.
Even so, I suspect you are on the right track with the loads you mentioned, I don't think I'd push it too hard.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra