Likely a little less than a 10% increase BUT that is assuming the heads, intake and carb are not maxed out in flow with the 302.  If there is a restriction than the HP increase will be less.  With all that in mind HP is a function of RPM.  The bigger factor is all the low end torque you will pick up with the extra displacement.  Give me all the specs of the motor and I will put both in my digital dyno for you

Horsepower is kind of meaningless really.  What you gain is torque, particularly low end torque.  If the heads, cam, intake, etc. is "too small" for the 331 vs. the 302 you may not realize max horsepower (torque relative to engine speed) because you won't be able to wind it high enough.  I would tend to agree that the gain is probably 10%, but 10% torque at low engine speed is significant. 

Consider the newest ones are over 20 years old at this point.  Sadly since '96 Ford went modular and never looked back.  Within a couple years the pushrod 5.0s and 5.8s were gone from every product line.