Craig,

I had the same problem, I must have dropped my A/T 3 or 4 times before I
really started thinking about ways to stabilize the tranny on the jack.
I have a small wooden framed wheeled cart that movers use to move
furniture and heavy boxes. I used that and some climbing rope to secure
the tranny to the jack. I used a cross piece of 2x4 lumber to allow a
nice level brace for the jack to hold to, and then very slowly jacked it
up. Now, I had my wife jacking the jack so I could concentrate on
aligning the studs with the holes. I think I could have done it alone
once I got the little cart made for the tranny. It just gets a little
wobbly once you get it high on the jack.

The main thing is to secure the tranny to the jack, it make a huge
difference. Take a trip to the local home center, or lumber yard, pick
up a couple of 6' 2x4's and some nail or screws and start designing your
own tranny cradle. 

I attached a PDF to this email for you Craig, it is a 3-D drawing of the
cart I used, please forgive the crudeness of the tranny in the pictures,
but it gives you an idea of where it sits. I used the 2x4 on the bottom
of the cart as my jacking point. I also used rope to tie the tranny to
the cart.

Kind of like what Jonathan wrote:  (Jonathan Farrugia
<jfarrugi@UMICH.EDU>) 
build a wooden platform, then mark the center and build a little "box"
on the bottom for the round jack pad to fit into.  if you want to get
fancy you can make some straps to hold the jack pad in the little box,
think metal plumbers strapping.  whoops, looking at your pictures i see
your pad comes off the jack.  i would build the platform and counter
bore a hole in the center for a big bolt, then bolt it on.  you could
use a smaller bolt with some large beefy washers.  i built mine out of
pine from shipping crates.  i have pulled complete drive trains as well
as transmissions with it no problem.

Good Luck Craig, and don't give up!

Andy