If it hasn't been a problem in the past, it won't be in the future. 50 years is a pretty good test.
I am also pretty sure that when the home was built the header was tight, but shrank when everything dried out over time. It happens in every wood framed house.
If you choose to, you can add a shim in the middle or in two places and snug it up in place, but that would at the most. I would think that the sheathing on the outside is helping to hold everything up, which in reality, isn't a whole lot of weight over that window as long as there is no snow load.
I would think the rafters are situated so that they are near the ends of the beam, not in the center so that minimized the use of the header.
If you fastened everything up already, be prepared to undo it. At least there is a presumption I am making that the old framing gave you the room you needed.
You may not need to undo everything, but I do see the common (outside) stud gone. The first thing is and this is a guess (that is, after you removed the damaged framing, you set the jack stud first, then the common stud last) on how you replaced the framing. The jack stud beside the window needs to be toe nailed into the bottom plate on the old marks left by the original jack. Making sure all the splinters, trash, old insulation is out of the way or will not move back behind the jack stud before it is set in place and secured. A lot of shaking goes on and things that are loose can go in places and keep things from going back in like they should. Nails from the old sheathing into the studs you removed are a good one for this. After all that is assured, finish nailing the jack to the header, insuring the face of the stud is flush with the original header. A little behind it is not a problem either, more room for drywall. Besides, the old framing you have may be 1 5/8" thick, instead of the 1 1/2" thick you have now.
As a footnote, check the wall with a straight edge to see which framing is out of line, creating the problem. Just a mention... it may help you understand a bit more of what is going on. If you used straight studs, good. If you used new framing that is bowed from the start, not good. Again, these things are to help you understand what you have, I can't do anything from here but make suggestions.
Once you have all the things done I mentioned above, and the jack stud is in place and the straight edge has confirmed everything to be flat and the jack stud should be showing hopefully the 1/2" you need for drywall, if so move on with adding the last stud and drywall.
If you are willing to undo even more nailing before you try the above fix, I see the header is slightly proud of the face of the stud beside it. If this means there is room behind the header this will help move the jack stud back when the header moves back also. I see perhaps an 1/8" of the 1/4" gone that you need. After thatr has worked hopefully, toenail the jack stud so it draws the jackstud back to expose the remaining window jamb you need and tighter still to the common stud.
This is also another presumption that the window, since so much has gone on around it that the window has not simply popped out of the rough opening a bit since the fastening has been violated by the replacing of the jack stud, meaning it only needs to be driven back in from the outside using a block of wood to protect the window from the direct hammer blows and refasten it in place so it doesn't pop out again.
Best Answer
I wouldn't mess with your kings. The biggest weakness in a wall of windows like this is stiffness. You want as much lumber going full-height as possible.
Also, that's not a header. It doesn't carry any load and merely offers window flange mounting and horizontal stiffness.
Pull the jacks, double the "header" (mullion, really) and toenail it in well, and carry on.