it weighs about 63# per 12"
12" by what, you bed depth or what?
Water is 63.2 # per cubic foot. That means your mulch would ALL SINK if thrown in water since much of bulk mulch volume is air.
Only domestic wood I've ever seen sink is just felled late spring (May) shagbark hickory with the bark removed. Cut and skinned some SBH one time in IL 60 years ago to float across a lake and most of it SANK.
Would guess in bulk chipped would be under 40# cu ft.
Chips or mulch, 30# per cubic foot is very heavy side of weight, sopping wet.
A more typical chip/mulch weight is 15# cubic foot, or 400 pounds per cu yard.
If your dump bed is about 40 sq feet, a foot depth is 600# or so - did you leave a zero off original post and talking about depth of chips in the entire truck bed?