Tranny drain and filler are on the right side of the car, both with square-headed plugs. Just undo the bottom one and the old fluid comes out. If you have OD, there's a separate drain for that on the bottom -- for a J-type, it's just forward of the solenoid.

To fill it, your life will be much easier of you spring for a pump at your local auto parts place -- they're a coupla bucks, and come in at least two sizes to screw on to whatever size container the Redline comes in. Pump it in the upper hole until it starts to run out. If you have OD, let it settle for an hour or so and then top it up again.

Your idea for draining the rear end is fine. Alternatively, I've also used the same pump with a longer piece of hose to pump out the fluid through the filler. Refill is just like you think it is. I use Castrol Hypoy C in the diff -- read what it says on the Redline container; maybe that's good back there too.