So you realy want the stored result to include the padding? See the tiddlywiki.com select widget example and set the value differently from the option display.
Here I only save the “tag name” not the modified and padded value.
<$select field=test default={{!!test}}>
<$list filter="[tag[HelloThere]]">
<option value=<<currentTiddler>> style="font-family: Courier" ><$text text={{{ [<currentTiddler>pad:suffix[50],[ ]addsuffix[test]] }}}/></option>
</$list>
</$select>
However I like to make the code easier to read with functions.
\function selection() [<currentTiddler>pad:suffix[50],[ ]addsuffix[test]join[]]
<$select field=test>
<$list filter="[tag[HelloThere]]">
<option value=<<currentTiddler>> style="font-family: Courier"><<selection>></option>
</$list>
</$select>