The left hand freewheel can either be held on with a weld tack or a loctite style product. The hard way to do it is to disassemble the freewheel, screw it on using the internal shelves to grip on and do it up REALLY hard, then reassemble the freewheel, from the back, on the bike... Can be done, and is what I did, but really do not recommend! :roll:
As for the crank, you will need to weld your chainwheel to the lefthand side, not just swap the cranks or you will have pedals undoing on you. If using a single piece crankset you will need to cut off the pedal bosses, switch sides and weld them back on... OR, cheat and just use more of the loctite on the pedal threads...
To sum up, yes it can be done, it takes some work, but can be done.