If you don't want to modify the Scan.conf file, you can remove the cog wheel so that the paper guides can slide independently.
This allows you to position the paper guide closest to the back of the scanner as far as it can go towards the back of the scanner. You can then slide the other paper guide up to meet the bottom edge of the stack of drawings to keep them aligned.
To remove the cog wheel, remove the paper chute from the scanner and remove the screw that attaches the cog wheel to the paper chute. After you remove the cog, the paper guides should be able to slide independently of each other.