1st gear problem on 3sd

[70c/10barebone]

I've had this problem for awhile and it's getting worse. Whenever I'm stopped and moving to 1st it's a fight. Usually its easier to force up to reverse then throw it quickly into first, que loud hammer sound. If I go to 1st while still rolling then no problems. The truck is a completly stock 1970. Is this a clutch adjustment problem? What do you all think? Feel free to ask for more info on specifics. It won't happen every time but becoming more frequent.