Yes, your FN Belgian '98 is more than adequate for the 6.5x55 Swede. I suggest that if you are building or having this built you chamber with PT&G's 6.5x55 Match reamer. It differs from the SAMMI spec reamer only in throating, it provides the perfect throat for seating 130-140grn bullets near the lands while still feeding from the magazine and can be rented here http://www.reamerrentals.com/SearchResults.asp?Cat=362
. The SAMMI spec reamer was designed for 160gr round nose bullets and by the time you get seated out to the lands you won't be able to feed from the mag. The FN '98 is recognized as the best commercial Mauser action built and will make a fine rifle. What stock are you planning on using?