Not sure that a properly proportioned stem will make a big difference on a non-proportioned frame :)
Do you have any example of a frame with these proportions?
Not found any off the peg at that size. Competitive Cyclist calculator seems to think a 59cm ST and 55TT and 100mm stem is around right, and from my riding position currently on a 58cmTT with an 80mm stem, I'd say it's about right.
Not sure that a properly proportioned stem will make a big difference on a non-proportioned frame :)
Do you have any example of a frame with these proportions?