I'm in the midst of doing so now, thanks guys
This is actually the first automatic trans car I've ever owned (and I'm getting old now), so I guess even if all auto trans are filled while running, I'd never have known it. I guess it's karma for second-guessing the service manual, too.
My initial check (fill to overflow) with it running at around the right ballpark temp, I ended up adding about 1.5 quarts to get it up from the "engine off" overflow level to the "engine on" overflow level. I have no doubt that's enough to cause problems.
It took me a while to swap bottles and whatnot and by then the temp was about 10 degrees past where it should be. So I capped it early while it was still flowing out a bit, and now I'm letting the car cool back off again so that I can do the final overflow level-setting at the right temp. Later tonight I'll go for a midnight test run to Denny's or something and see if it's still slipping in 3rd.
Also, I finally got the Z1 trans mount installed earlier while the car was cooling off the first time. Really, really easy to install, even on a stock car with everything attached. You just need a way to get the car up in the air (normal jackstands at a mid-high setting would work), and a bottle jack to
hold, not truly lift the rear of the trans itself. The rest is just 8x fasteners to drop the crossmember, drop out the old mount, then reinstall it all, drop the bottle jack, drop the car. I'll see how that feels on my midnight run as well.