This could not be easier. You only need two accessories:
1. A straight stitch needle plate.
2. A straight stitch presser foot.
I found them on eBay for my machine, actually for a slightly later model, but they both use the same accessories. My machine is a 337 as I have mentioned before, the parts I got are for a 348.
It is only a matter of removing the zig-zag presser foot, pulling open the bobbin cover, and removing the zig-zag needle plate, then installing the straight stitch plate and foot in the opposite order. Of course if you need to use zig-zag stitches you have to change things back to the zig-zag configuration.
Does it make much difference? I found the stitches are straighter, lacking that slightly slanted look, and the material does not get pulled slightly into the needle hole. As a result the machine seems to sew more smoothly.