If you want to do it safely...get the correct tool, then you have very little chance getting hurt while doing it. Using a big C-clamp...yes you can do it that way, but would it be worth the pain if it happened to slip? Personally, I would just crank the torsions you have now and see if you get the required or desired lift out of it...changing the keys still puts the same stress on the bars.
i took a metal plate cut it out to fit the top of my jack, and welded a 1/2 inch bolt to the top of the plate, place it under the key and jack it up! I have my truck on jackstands, of course! Ive done 4 trucks with no problems
Taking them off with C-clamps is the easy part. Loading them is a b*tch! My advice would be to buy the tool, use it, then sell it. I've done it plenty both ways and that tool makes a world of difference.
I put a pipe wrench on the torsion bar and jacked it up to take the tension off. Worked very well and with a good pipe wrench not much of a chance of it slipping, then with the bolt and keeper out you can slowly let off the tension.