Transmission Fluid

Needed to purchase tranny fluid for turbo 350 in my 78. Back injury has kept me out of driving her for a long time.. Quite a bit has weeped out, none on the dip stick. What do you recommend? In the past I have used Penzoil ATF Dextron lll for older vehicles. Cannot locate in any of the auto part stores near me. They recommended Dextron VI. I didn't purchase any because I had to many questions they couldn't answer straightforward enough.

What's your thought's on this?

Thanks,

Kenny 78