I had assumed you were more familiar with 3dcalc.

Your mistake is twofold. First, you used '-a' to define the input, whereas in my formula, I used the variable name 'x', so you should use '-x' to give input dataset name (or change the 'x' in the stat2cdf function to 'a'). Second, you multiplied the function calls by 'a', which is not write. You could try this

3dcalc -a 3dttest++FACES_Csim_WB+tlrc -prefix 3dttest++FACES_Csim_WB_ZtoT -expr 'cdf2stat(stat2cdf(a,5,0,0,0),3,44,0,0)'

If you do not give a dataset definition for variable 'x' (with the '-x' option), then the program uses the x spatial coordinate for the value 'x' in the expression. For voxels a long ways from the brain center, x can be large (60-70), so no wonder you got ginormous values out!