Howdy-
You can add either of these options to suppress the commented text descriptions, and only output numbers (from the 3dClusterize help):
-nosum :Suppress printout of the totals
-quiet :Suppress all non-essential output
Note that even if you don't use the -quiet option, you can use 1dcat to dump the text from a file excluding the commented stuff-- this might be my vote, because you can have an output text file with commented description, file info, etc., but still get just the numbers out from, rather than losing all that lovely descriptive info.
Summary:
To just get numbers, you could do:
3dClusterize -quiet ... > Cluster_numbers_only.1D
*or*, you could do this and have the best of both worlds:
3dClusterize ... > Cluster_full_table.1D
1dcat Cluster_full_table.1D > Cluster_numbers_only.1D
--pt