a couple of strips of of 1/8 or 1/16 white plastic will work as runners. Make them long enough to get a good grip on with a line or vice grips in order to remove them.
If you have an opposing wall within a reasonable distance, you can set a 2x4 on edge against the opposing wall, then use a bottle (hydraulic) jack to push a 4x4 against the safe. It will be tedious, but you can do it.
You can do it this way w/o runners, but might scar the flooring.
I wouldn't put drywall under the safe. If it's sitting on flooring over concrete, you won't have fire exposure there. The drywall will come apart over time with the weight sitting on it.
Edit--just reread the bit about the water pipe. DON'T put the safe somewhere you will have to move it to work on the pipe. The universe is constructed in such a way that this will cause you to need to move the safe repeatedly.