I would actually suggest doing both via editing the theme than with adding css. While the css option is much quicker, editing the actual files will give you less issues down the line.
Specifically, the ajax portfolio uses a percentage wide grid while the single page uses pixels which get adjusted based on the screen resolution (responsive). So setting the grids to a static size may have some odd effects.
If you just want to do this change, you can edit it in includes>loop-portfolio-single.php. Search for these individually:
<div class="eight units alpha min_height_1">
<div class="four units entry-content">
Since this is a 12 column grid, the slideshow is using eight and the content area is using four. So you could just reverse them by changing the eight to a four and the four to an eight and have pretty much what is done in the ajax portfolio.
If you want to continue on and change the ajax portfolio too, you can do the same kind of thing in functions-flagship.php. Search for:
echo "<div class='inner_slide'>";
Just below that are the two divs that contain the slider and the content. You could remove your css change that you have so far and instead just switch the classes(two_third and one_third).