If its a shoplight with a cheap "residential use only"
low-power-factor ballast, using anything but T12 40W bulbs
will very likely cause the ballast to overheat & turn off
after running for awhile - once it cools down it will turn
back on (but leaving it this way will eventually destroy
it - the overheat protectors are generally cheaply made
and after many on/off cycles will get stuck "on"...which
will result in smoked ballast (horrible smelling stuff btw))

If its a better quality T12 ballast it may work just fine :)
(esp if you use 1 T8 & 1 T12) And yes the T8 will be
overdriven some (you might shorten the bulb-life a bit, but
the extra light output is nice) - I ran 1-T8+1-T12 in a T12
shoplight (with commercial grade ballast) on a fishtank
for a couple years...great light output on the T8, so much
brighter than a T12 (bulb still worked fine when i shut
down the tank - sold the plants & moved the fish to another)

The cheaper ballasts are generally around 6" long (and will
probably have 'residential use' printed on them).
The better/commercial ballasts are generally around 8" long.