I like them, Where would be the best place to mount them?
You'd have to play with that a little bit. It depends on the available space you have in the bed given the wheel wells, and how accessible you want the cans. Do you want to be able to get to them via the tailgate or not, do you have a shell mounted on the bed that would stop side access?
The holders can be mounted upright or on laying down, which changes the foot print. They can also be mounted as singles or in banks, so depending on the space available and the number of cans you want to carry there could be various scenarios e.g. a single bank of 4 cans, two banks of two holders, one bank of two and two singles etc.