Installing downtube shifters with no clamp
Without any additional knowledge on the bike, my advice would be don't drill into the down tube. You run the risk of making it structurally unsound. That being said, some bikes with downtube shifters have a hole drilled in the downtube with a rod going between the mounts and maybe some epoxy using something like this:



Some other options are:


Find a different clamp (the Problem Solvers downtube shifter mount is bigger than most, has a 31.8 mm diameter)
Try making your own clamp (e.g. with a hose clamp; this may be necessary if you have oval/other shaped rather than round tubes)
Braze on some downtube shifter mounts (Shouldn't be too hard for steel frames)
Switch to using bar end shifters/brifters (Shimano makes a Tourney 7 speed brifter)/Retroshift levers (which mount a downtube shifter on the brake lever)




An aside:

As noted in various other questions on this site (do a simple search), its often not advisable to convert a mountain bike to a road bike or vice versa due to cost and geometry reasons.