I need to remove my transmission for a rebuild. A couple of simple questions.
I need to support the rear of the engine right? What would happen if the support slipped out? What would be damaged, motor mounts or what? What other ways is there to support engine other than a jack. I've removed the engine and transmission together, but not separately. what worked best for you?

I've always used a floor jack with a block of wood under the oilpan. After the trans is out you can probably let the engine gently rock down and it will rest just fine. I wouldn't, however, let the extra weight of the trans hang on it without extra support though.

One other issue. Check for clearance between the distributor and firewall and also between the fan and radiator when the engine is tilted back. You may want to remove the distributor cap before you start. I have to on my car.

Regards,
Gary Anderson

What Gary said.

If you have tight clearance between fan and fan shroud, might want to keep an eye on that too.

after the rebuild remember to get a buddy or helper to help put it back in,, bolting a manual back up to an engine can be tough enough when its out of the car. underneath is like trying to mate elephants : )

Joe - If you don't support the engine remove any pump mounted fan. Also check your distributor clearance. I've had mine out with supports under the pan and without. As long as your mounts are good it can hold the engine by itself fine. I found having the engine tilt back a little made getting them mated up again a little easier, too much tilt and you'll be struggling with the shifter clearance in the tunnel.

It's no big deal putting a manual four speed back in place from under the car. I've done it a bunch of times by myself. I just balance the transmission on a floor jack and guide the input shaft home by hand. Once the input shaft is through the clutch disk, it's easy to slide the transmission the rest of the way in. I usually install the shifter after bolting up the trans, but before jacking it up and installing the cross member - one less thing to snag on something.