One way to test if your clutch is not fully disengaging: With the vehicle stopped and idling, transmission in neutral, depress the clutch, wait at least 3-5 seconds and attempt to put the transmission in reverse, if the gears grind/chatter the clutch is not fully disengaged. If no issues, you're clutch adjustment is probably good.
Make sure you have the proper lube in your toploader. The following recommendations are consistent with my experience, although copied from a mustang website.
As far as lube preferences go, the toploader experts suggest staying away from
synthetic lubricants as they tend to hinder the function of the brass blocking rings (which need to have some “grab” for the transmission to shift properly.) Therefore it is best to stick with a conventional 85-90wt lube.
Also, if you are using a Hurst Competition Plus type shifter, you might consider freshening it up at this time in order to get better shifts. For details of how to clean it up, I suggest the following tech article:
http://www.fordmuscle.com/forums/tra...t-shifter.html
Hope it's an easy fix. John