when I bought my 74 240 years ago it did as yours I put a new clucth master cylinder on and it then shifted smooth as margarine ,but it then blew the slave cylinder first time out. I had to limp home shifting without a clucth.After replaceing the slave cylinder the clucth chattered as brake fluid from the blown slave cylinder had saturated the clucth ,I took the slave cylinder back off and sprayed a can of brake cleaner in through the opening ran it a few miles and it now shifted like new. Moral if you change master change slave.....
William Rogers.......