Full extension rails are probably best going to come from the original vendor as a general principle, rather than attempting to use universal rails.
If you have a wall mounted rack, unless your walls are not drywall, physics is working against you. It's already a pretty intense heavy cantilever, and putting a server in there that can extend past the front edge is only going to make that worse.
If you want to use full extension rails, you should get a rack that can sit squarely on the floor on either feet or appropriately rated casters. You should also make sure your heaviest items are on the bottom ESPECIALLY if you have full extension rails - it will make the rack less likely to overbalance itself and tip over when the server is extended.