The book only showed three taps.
You need the 8 inch duct to even come close to your required CFMs, and that is if you are lucky enough to have .5 TESP.
Do not waste your money on a service call unless the guy knows how to check all your ESP readings like Dash said earlier. Like Doc said also, you are probably oversized on your heat with that furnace. They make a 070-08 furnace. After glancing into the Manuel J book, I see you have a winter design temp of 17 there in Richmond, VA. Not knowing what the heat load is on your brick structure, I would still guess you are close where a 45-08 furnace would work for you.
After reading your other thread, you really need someone who is knowledgeable to come setup your system correctly and check everything out, heat rise, combustion, refrigerant charge since you have messed with the blower speed if it is a piston metered system, CFMs.