Tomy: The flat side 54 bed will fit but there's one problem, the frame rails over the back axle of your 48 kicks up higher than the rails on the 54 truck because when they flattened out the bedrails in 54 they made the bed deeper so it would a comperable size. Here's a pic picture I made several years ago showing the difference.
http://www.chevygmcofkc.org/Gallery/data/3002/1FrameComparison.jpg There are a few different option.
1. You could modify your frame rails (not a good idea).
2. Let them stick through your bed floor (Ugly, but I've seen it done).
3. Put bigger wooden blocks between the frame rail and bed to raise it up higher (Not bad but might look funny and the splash pan between the running board and bed in front of the back fenders won't reach).
Might be some other options but those are the one's I can think of this late.
Mongo