First thing is to make sure you are still measuring the initial velocity at 8 minutes. To do so, either run the continuous assay or run timepoints and check that the variation of absorbance is linear towards time (Abs=f(time) is linear). The initial velocity is the slope of this curve (expressed in sec-1 or min-1). Then convert it into concentration per time unit by dividing it by the extinction coefficient (so it's now in M/min), then multiply by the volume of the assay to get it as mol/min. Now, knowing the protein concentration of your stock solution (mg/mL I presume), you can calculate the amount of protein introduced in the assay in mg. So now dividing velocity by this amount of protein will result in activity expressed in mol/min/mg which is the enzyme specific activity (if the enzyme is pure in the stock solution).