You could use the same amount of pale extract as you would have if it were extra pale. Your gravity should be right on, but the beer may be a touch darker.
Alternatively, there should be a way to use some pale extract, and some corn sugar to keep the color in line too. I dont have ProMash here, but some one with the program could figure this out.
I would use the extract that you have, and not worry too much about the color.
EDIT: It sounds like your plan to pitch the new wort on the old cake is spot on.
Planning: New brewery at new house!!
Primary: Barley Wine
Secondary: Red Ale, Pale Ale
Drinking: Brown Ale, 75 min. IPA, Oatmeal Stout, Poor Richard's Ale, Chocolate Cherry Stout