The letter tool unfortunately doesn’t have the full editing capabilities available for invoices/estimates. Although there is an active feature request to add CSS editing capabilities in the Advanced Customisation tools.
I’ve not tried this but you could add a large white space to the right side of your logo, thus pushing it off to the left. Admittedly it’s a bit of a hack!
The textual content of the footer can be modified in the Letter settings area. The footer also supports some basic HTML markup. Wrapping the text in a <font size='1> tag should work.
<A HREF="...">
<B>
<BR>
<CENTER>
<DIV ALIGN="..." STYLE="...">
<FONT SIZE="..." FACE="..." COLOR="...">
<FONT STYLE="...">
<I>
<P>
<SUB>
<SUP>
<U>
<S>
Yes this is very simple to do, just edit the text or replace it in the main letter editor page.