This is a semi-typical SHO problem...
Try to shift into reverse with the engine off, if it's still hard, you might have tranny problems. If it goes in easy, your clutch might have been installed wrong. The SHO has a self-adjusting clutch quadrant, IIRC, that may not be functioning properly, thus not allowing your clutch to fully engage, but you would most likely experience grinding in other gears as well. To "help" the clutch adjust, pull up on the pedal with your hand using 10-15 lbs of force, if you hear it click, you've found your problem. Or at least part of it.
There are people here who can explain the cause of your syptoms better than myself, but my 89 will grind every time into reverse due to brass blocking rings that get worn out prematurely. To avoid this try shifting into 5th before reverse. I have to do this each and every time or wait 5 seconds with the clutch in and even then sometimes it will grind a little.
Hope this helps you out...
~Ben