iTunes truncates MP3s. I know the fix, but I want to automate it with AppleScript
Figured it out!

Here's the relevant line: set newTrack to add location_in_finder to library playlist 1. I hadn't realized that add track returned a reference to the track added, but the iTunes AppleScript dictionary showed me the light. 

The Final Script is below:

global okflag, selectedTracks, newTrack
set separator to "."
set okflag to false
-- check if iTunes is running 
tell application "Finder"
    if (get name of every process) contains "iTunes" then set okflag to true
end tell
if okflag then
    try
        tell application "iTunes"
            copy (a reference to (get view of front window)) to thePlaylist
            -- check if "Library"; later, we will not offer to number a Library playlist
            set using_the_Library to false
            if (class of thePlaylist) is library playlist then set using_the_Library to true

            if item 1 of selection exists then
                set using_selection to true
                copy (selection's items) to theTracks
            else -- its the whole playlist
                set selectedTracks to (get a reference to thePlaylist)
                copy (thePlaylist's tracks) to theTracks
                set using_selection to false
            end if

            repeat with i from 1 to (count theTracks)

                if using_selection then
                    copy item 1 of selection to thisTrack
                else
                    copy track 1 of selectedTracks to thisTrack
                end if
                -- thisTrack now set
                copy (get thisTrack's location) to location_in_finder

                delete (some track of library playlist 1 whose database ID is (get database ID of thisTrack))
                set newTrack to add location_in_finder to library playlist 1
                play newTrack
                delay 1.5
                stop

            end repeat
        end tell -- iTunes
    on error error_message number error_number
        tell me to display dialog error_message &amp; return &amp; "Error number " &amp; error_number
    end try
end if -- okflag