As an OHV owner that's done this job (same as pulling a distributor), just confirming what that other guys said; you need an alignment tool to get the job done.
NOTE: Don't pay more than $15 - they are all the same. Get a HIGH VIZ color i.e. Yellow.
In addition to what Mbrooks420 said, if I remember correctly,
you need to install the alignment tool on the old synchronizer BEFORE removing it, marking an "index mark" on the rear firewall where the Arrow on the alignment tool is facing.
Then, remove the old synchronizer, remove the alignment tool, and place the alignment tool on the new synchronizer.
As you place/feed the new synchronizer into the block, the gear teeth on the synchronizer will mesh with the gear teeth on the end of the camshaft. As that meshing occurs, the alignment tool arrow will slowly move towards the index mark you made on the firewall.
More than likely you'll have to remove/insert the new synchronizer a few times to hit the index mark exactly - as you'll be a tooth or two off in either direction the first few attempts. It's not a hard job, just a PITA due to the lack of good sight, space, and having to hit the mark spot on. If I remember correctly, it's easiest with the hood off...
Camshaft Synchronizer Alignment Tool Kit Spectra TK01