its actually 590 CFM btw. And Flowmasters dont flow much better than stock mufflers. Very minimal performance gain. I would go with a set of spintech'sPresuming that the flomaster system was hooked up to the stock manifolds and stock cats (the Tri-Y's and 570 CFM cats replace these pieces) you should not have a problem since the flomaster system should already "end" in the right (stock) place.
If something else was done, a good muffler shop should not have a problem joining the two systems.