And in addition--check your clutch linkage and maybe try tightening it up a bit.
It is easy to do (at least on a 66 GMC), just crawl under it and tighten up the nut with your fingers. My experience was there was some slack in the linkage and I tightened up just until the slack is out--no more problems.
Finally, come to a complete stop before bringing into 1st gear.
