A 28 and 31 spline diff cost about the same and sometimes literally the same. Basically if you go 5 lug 31's are a better choice, since you are buying axles anyway.
If you aren't using a drag radial or slick, you'd probably never break a 28. But it could happen.
That is an accident waiting to happen, you'd think a guy savvy enough to get his car that fast would be smart enough to use better axles.
The axle doesn't have to break before the car starts moving.
personally if you are sticking with 4 lugs then just buy a 28 spline moser or strange axle. Why add the cost of getting a 31 spline carrier perhaps the cost of rebuilding it and the cost of re setting up your gears. Easier to buy a set of high quality aftermarket axles. 28 spline. Provided you are not a 9 second or quicker car.