I just put a new 512 in my 72 yesterday without a helper. Left the transmission in the car when I pulled the engine. I've never done it with the tranny attached, seems a lot more difficult with the cherry picker and shop setup I have, but considering how many people like doing it that way I probably should consider it next time because I'm probably wrong.
I think it's pretty easy to line up the tranny and engine. I slowly lower the engine down until it's over the mounts, put in the mount to K-frame bolts, tighten the mounts to the block, and while it's still being supported by the cherry picker with a load leveler (cheap at harbor freight and makes things WAY easier), I use a floor jack under the car to support/raise/lower the transmission. Unbolt the tranny to crossmember mount so it moves. If you put it at the right angle, you can shift the tranny forward/rear/left/right by rolling the jack a tiny bit, and up/down by pumping the handle. Being able to move both pieces has made it really easy on the handful of times I've done it. Others have way more experience than I do though.
I've never put it in from the bottom, that seems to be the easiest way if you have the clearance and space to do so.