The big issue you are going to have if you NEED to have 300px width sidebars is the whole thing is part of a grid system where the sidebars are actually in a 290px wide grid of 4 columns and the content is 8 columns making up a total of 12. If you start to mess with that it tends to break the whole layout and all the fixes that rely on the grid. You would have a much easier time just using sidebar content that is 290px wide and then removing the 30 pixel padding and margin on the sidebar. Or, using content that is 260px wide and not editing the dimensions at all but that is a bit more restrictive.
Really, we wouldn't be able to walk you through modifying the layout that extensively and still being able to keep everything responsive and have no issues cross browser.
You can modify the loop for the blog in includes>loop-index.php. So perhaps right after:
Depending on what you want to do and where you want it to be included on each post.