Stock springs can allow back pressure to force valves open and be struck by the piston. As you can imagine this would be catastophic. Some brakes, BD for example, have reduced pressure brakes that can be used with stock springs but they also are less effective. IMO you should change to 60# exhaust springs regardless which brake you install.
Why are you limiting yourself to Jacobs? They're good but not the only one available. I've had BD and now Jacobs and both are good but so is Pac and probably others, and at less money. Try e-bay for bargains.
Dan