Steve,
That transmission has a syncro drum which has identical internal components on each end of the drum and the method of engagement in both second and third. Having said that if one gear won't engage or stay in gear it is very likely the syncro is worn. The rings on either end wear and could cause the problem you describe. Another problem could be in your floor shifter adjustment or wear in the shift arms and/or the 2-3 shift fork internally. When you get the grinding sorted out you should get the OD hooked up and be able to do 70. All of the electrical components are available and it makes for a better driver.
Fred
