There will be a lot of extra heat buildup, and the mounting hardware will have to be adjusted accordingly. We mount a similar PT-7 in the Tode guitar amp this way, but there's a lot of air space inside the Tode, and giant air vents just above where the transformer sits.
I'm not sure I understand what doesn't need to be painted by mounting it this way. The lamination stack is varnished already, and doesn't need to be painted.
-PB