TH350 Troubles / Questions

[YourBuddy'sTruck]

Having trouble with my TH350.
1970 C20, 350cc, Th350.
Here's the story.....
Started leaking out of the pan over the Winter.
Drained fluid, pulled pan, new seal, then refill with new fluid.
Ran fine, everything okay.
I don't use it much in the winter...literally 100 miles or so the whole winter.
As we've warmed up, I'm driving it more and now the transmission seems to be slipping.
A little at first, now it's quite a bit of RPMs till it finally shifts and gets up to speed.
Once up to speed, seems fine.

The fluid I put in is Dexron-VI / Mercon LV.
(I don't even know what these words mean, but that's the one most of you here suggested in threads)

Did I do something wrong? Or is this thing in need of new friction rings and a rebuild?
Any help / advice would be appreciated as I know just about nothing on Transmissions.
Thanks in advance.

Also, last year I replaced the Actuator (vacuum shift) Valve with your help, as well as the Speedometer Cable and Gear.