This article explains how to use High Density Diskettes in a Tandy SL with a 360K drive. This is done by fooling the Drive Controller into thinking that your 360k Disk drive is a 720k disk drive.
Note: This does not require any permanent hardware modification to the system, however if done to the A: drive, you may not be able to read the DOS diskette or normal 360K bootable diskettes. It is suggested that this change be done to the B: drive only.