Are you going deep or shallow on your sand bed? IMO 60 lbs of substrate (not live sand, but dry bags of argonite) are plenty for a 48"x18" tank. My sand depth probably ranges from 1.5" to 3". If you're going deep sand bed, then you'll need a lot more.
I just put in 200lbs in my 90g which works out to a 4.5-5" bed...
However, I'm now second-guessing myself, and thinking that a DSB is not a good idea after reading all the DSB maintenance/crash debates. Now's the time for me to pull the sand out if I want, but I can't decide.
If I do pull it out I'd probably go with 2-3" in the display, and a "DSB in a bucket" as opposed to keeping it in the main display.