Actually just got done with something similar this winter but with a sport chassis bed .  Many metal dedicated hand held options and wheeler metal has what you need.  The aluminum is a different critter but the tooth blades are what you want . The hot saw will be a disaster just fyi.  You should be able to get the depth of  cut you need with a hand held one with teeth.  There are dedicated grinder disks for aluminum and they won’t slick over like the regular metal ones.  . Id like to find a project like that .