To remove the attribution footer, search in your css for 'attribution' and use html comments to comment out those sections. Check with preview to see if they are gone. When you click 'save template' blogger will ask if you want to delete the attribution gadget. You can delete it, and it will stay gone.
Next, making the post wider. Blogger is stuck being optimised for small screen sizes no-one uses any more. To increase the post width, change the 'value' attribute of this tag (search for 'content.width'):
<b:variable default='930px' name='content.width' type='length' value='1000px'/>
And to change the width of your gadget panel, change value of:
<b:variable default='360px' name='main.column.right.width' type='length' value='370px'/>