Run the engine with the trans in neutral for a minute right before you pull the speedo assy.
This fills the converter so the fluid level in the trans pan will be at it's lowest level so not much fluid will come out.
In neutral because the stock valve body transmission only circulates fluid in neutral.
The lift the rear and do it to it, and you should only loose a couple of tablespoons of fluid that sit in the case indent where the assy goes.
Myself I would jack up the back, fire it up and let it run in neutral for a few seconds, shut it off, then yank the speedo.
Those seals only take about 2 min to install, so no worries there.
Excellent advice to just make it that much simpler.
You can also note the numbers by the retainer fingers and put it back the same
Yep, run it to get the fluid as low as it can get in the pan first.
