Trans issues

[RustyHunk-O-JunkGuy]

If anybody has some ideas that would be great.
I got the "new" truck a couple of months ago and decided to swap in an automatic so the wife could drive it (no p/s or p/b). I figured I'd use the TH350 I had laying out back. "Hey it was a good trans and it's free!" Never mind it's been outside for like 6yrs. A basic Summit torque converter and a good clean out with a new filter later and we are in business. Back out the driveway head down to the gas station everything is going great. Go to grab reverse again and there is nothing, no forward gears, nothing. Shut the truck off, check the selector all good. Start it up and it grabs drive, make it part way down the street and it slips out. Shut it off and and it will get drive again, limped it home going straight through the stop signs because anytime you hit the brakes it slips out. I get it home, pulled the pan again and checked all of the spools in the valve body, everything moved and returned fine. Start it up and head down the street and it runs great and shifts fine! Three or four stop signs later and it won't shift out of first.Take it home check vacuum line, all good hop in again and now it slips like crazy again!
Next up I'm going to check the governer and maybe replace the vacuum modulator if that turns up nothing. Any ideas?