Cap the end, or better yet, just make the manifold. You say you want a SPS tank, so I assume you might be running GFO, carbon, a chiller, and/or reactor. Its easier to do professional looking plumbing when the stand is empty vs adding on with a full sump and all of the electrical in the way. Just my two cents.
I also like the return. I just did mine the same as your diagram. Just watch out for the valves. I noticed the inside diameter of my 1" valves is 3/4". In other words the valves can restrict flow. I think since you are splitting the flow to 2 different returns then you wont have to worry about the smaller inside diameter.