Start with the fluid, filter, and band adjustment. it sounds like low pump pressure might be the problem, and the filter would be the 1st thing to do.
You have not given us any idea how many miles are on the trans itself, it may be time for a rebuild. 100,000 - 125,000 is a lot of miles on the 67 drivetrains. Most didn't expect the vehicle to make it 10 years or 100,00k miles, the ones that did were truly rare. .