I am not a bagmaker ..that is for sale....I
I would suggest that the rectangles between the Strap could be removed and then moved down so that both rectangles can be sewn to the back of the bag..... I think the bag would sag in the middle less and... you would gain a shortening of the strap by a couple of inches...don't know if that is enough........
If it were me I would send it to one of the pros on here so they can do it without messing it up
Be sure to measure and specify how long/short you need the strap to be..... I would also ask him to replace the buckle with a solid cast or forged buckle of an early American design.
That's my $.02 ..... good luck