if you go too big, you will have throttle response issues. the duty cycle can get too small. I would go with 19#, unless you plan on NO2 or something. 19# injectors can support something like 215HP at the crank.
FI size = (HP * BSFC)/(Clyinders * Duty cycle)
BSFC = ~.045 for N/A
Duty Cycle = 85% for ford injectors.
as for what mustangs, I think any late nineties v8 should work. Not 100% sure. I got mine from a 98 v8.
right, but if you do the equation I posted, putting in 160 HP you get 14.1 lbs/hr. that's with perfectly clean injectors. If you spend money to allow more air into the combustion chamber, you will quickly run out of fuel. True, larger injectors BY THEMSELVES won't add any more HP, but they will allow your other mods to get more out of your engine.