I just went through the same issue on my 70. When its cold, does it start okay, then when its hot, it turns over slow or not at all?
Mine was starting fine when cold, but was acting just like yours when it was hot. I just replaced the starter solonoid last night and it started probably 10 times or more while I was out running around today. Once it kinda turned a little slow, but still started.
The solenoid is cheap and fairly easy to replace. If the battery and connections are good, then I would try replacing the solenoid. While you have the starter out, take it down to one of the parts stores that have a starter tester and have the check your starter out. That way you will know if the starter is good.