Hi.
It'd look no weirder upside-down than the originals do

.
Since the Smarvo chassis is a good bit wider than 19" (~18" actually), all You have to do is to find a 2U or preferably 3U 19" rack box, punch/cut/drill the front for the holes for the potentiometers, push buttons and jack, the underside for the pre amp tubes, the top for the power tubes (Power PCB is narrower than 17" IIRC) and the back for the output, DI etc. Then You relocate the transformers, and make a some sort of cage around the tubes.
If You find a deep enough box, You can mount the power section tubes-down as well. It'll be all right as long as You change the bear claws (IDIOTS) to top-retainers. In any case, You'll probably have stuff poking out from both sides of the 3U box.
Is it easy?
Yes.
Would it be worth it?
No (if You ask me).
Regards
Sam