Jack stands or good blocking under the frame. I've often used the frame extensions in front of the axle, even though they're higher then the rest of the subframe, so that I have lots of room to maneuver under the center of the car.
I've found cinder blocks and 4x6 blocks of wood are nice and sturdy for such things, and I trust it more then some of the spindly cheap jack stands I've seen in discount places.
You want to support the frame because as soon as the transmission drops out that's a couple of hundred pounds off the suspension. You'll also need a jack to help you lower the transmission. I've slid transmissions out from under cars on a piece of plywood after dropping them, they won't slide on the concrete too well.