I need to make a 3D firetruck grooms cake. I'm not really sure how to go about supporting it so its not sitting flat on the board. Any ideas how to go about this? Thanks in advance!!

To get the look you are trying to achieve you need to build a platform..What I did when I made my firetruck was first make a board..Then I stacked 3-4 pieces of 1/2 inch board and glued them together.Make this for the centre but it needs to be the exact length and width of the cake.I then glued that to the base board cutting out the wheel wells and covered the rise in between with black construction paper.Here is a pic...

I took the axles/tires off a toy engine and had my husband (a firefighter!) make the platform, which I covered in fancy foil to give it that shiny, chrome look.
He also put dowels behind each tire so it was really the dowels offering extra support--and also so the truck wouldn't roll away!
This was one of my toughest cakes--took many, many hours--and was super heavy!
Good luck--and be sure to post a photo when you are done!