I am looking at a 38 Fever with 500's and extension boxes my question is how do the boxes effect the ride on a 38 fever and what speed should I expect when I sea trial it?

Which boxes - Imco or Stellings?

The IMCO boxes on my 38 Fever helped it IMO - ran best with -2 shorties and P5X's. Hard to say what your bot will run with 500s, are they HPs, EFI's, fresh, tired, stock, what props, drive height...... I could go on but I"m gonna assume all stock and throw 79 out there.