The Smash, in its stock base with stock feet is 7" tall and 11.25"W x 11.25"L. I may be a little plus or minus on height as I have the Smashup installed. The 6SN7 is lightly shorter than the OD3. I guesstimated an allowance for that.
Hope it will fit. Great linestage and even better with the Smashup.
Cheers,
Geary