Well, to be perfectly on the safe side, you have to do some math.
First determine your altitude you will generally be around, with a margin of +/-125 feet. If you run any differen't nitrous shots varying from 125, for every one HP up or down add to altitude margin exactly 9.8*(square root of HP). For example, a 150 shot would be +/- 573.4 margin for your altitude.
Find the average and label altitude as a.
With 281ci in this particular modular Ford 4.6L, and with the Mustang GT stock cams (if this is a engine swap, i.e. crown vic motor, explorer, do NOT do this) you should be able to use this formula to determine if it is safe or not to use that particular nitrous shot.
(a+(281/(14.5*abs(a)*(-9.8)^2))^(a/73.235)
If the answer falls between 10-20, the coefficient of safety for the internals of this particular engine, you are safe.
Use a wet kit.