Based on what ground someone can make the statement that a 6x2x4 is "fine" "upstairs"?Not every house is framed the same (2x10, 2x12, 16"o.c., 20" o.c, etc), at least make sure that the long side is orientated perpendicular to the floor joists, direction wise. If the carpenter was cutting corners when doing the framing (which you don't know unless you were there) all bets are off. Reinforcing the floor (or at least getting a structural appraisal) seems the smart thing to do when adding something that heavy.