Hi Jeremy,

It is best that you transform the t values in your .1D file with p values using the program 1deval wich performs operations like 3dcalc but using 1D files for input. For help on the type of operations possible with 1deval, look at the help for 3dcalc.
Lookfor "statistical conversions" in the output of 3dcalc -help, you have all sorts of statistical conversions that you can make.

You will need to provide parameters for the conversions. These parameters are stored in the field: BRICK_STATAUX in the .HEAD file of the dataset from which you extracted the statistics.
Read about BRICK_STATAUX in README.attributes file and use the program 3dAttributes to easily pull these parameters out.

If you don't have a volume with the statistics (I suspect you carried out the statistical analysis on the surfaces directly), you will have to supply the parameters by yourself. For a T statistic, all you need is the degrees of freedom.

cheers,
-z