1966 3 speed manual trans problem. Help!

It should stay in gear but as far as shifting and grinding, That transmission doesn't have a synchronized first gear. At least my 65 Dart didn't. Like Dartman mentioned, there might be a linkage adjustment too. The clutch may get better with some use.
Thank you toolmanmike! Yes that's what I'm hearing that it doesn't have a synchronized first gear and this gives me hope that it could be something really simple! Clutch seem to be grabbing good. When I started going in reverse it grabbed with the pedal about 1 1/2 inches from the floor and easily spun the tires a bit. We have some bad weather on the way so I put it in the garage gently using 2nd gear. Thanks again for your help!