When I built up my bikes, I too had to deal with long arms, but fortunately I am more proportional. I have found that setting up the seat all the way forward, then checking my stem length will tell me what should be too short to start with. You can sort of feel the cramped cockpit dimensions. Then I move the seat all the way back to make sure I am stretched out a little. For you, with your mismatched dimensions, you should set the seat so that it feels reasonable for the pedals, then work the arm reach. You are basically going to be limited on the seat by the short legs relative to the arms. You may find that your frame geometry may not allow enough adjustment to do both. And as folks mentioned, long wheelbase/long chain stay/long fork rake bikes are your best best for having options. Touring bicycles are your friends. And they have pannier mounts to save your back from that ill placed pack. Which said pack is pounding your scranus into the seat and reduces comfort.