Assuming you don't mind the visual compromise of the case being sideways (as the bottom will be visible) plus any marking of the worktop or case because it will rest on its side, then yes, it is possible to use them in a vertical orientation but there will be a slight reduction in cooling performance. As the fins are designed for convection, having them in horizontal orientation is not optimal but the heat pipes will work in any orientation and can actually benefit slightly from being vertical. I would recommend going with a 45W TDP CPU as this will allow more headroom for the reduction in performance but you should still be OK with a 65W, especially if you take some time to optimise it for fanless cooling by undervolting and ensuring it doesn't boost too high and push the TDP over 65W.