You're talking quite a bit of weight: 50 - 75lbs. I assume by relay racks you're meaning a two post rack. I would suggest two options:
1. Get a center mount shelf and just set the server on the shelf.
2. Mount the server flush with the front of the rack, and at the bottom, and make a floor support for the rear of the server.
Are you short on space in the width, height, or depth department? There are a lot of options out there.