I don't know. It looks pretty stable to me. I might think about some sort of cross brace on the end but I kind of like it. A little more room that jack stands and a creeper. Car lots have been using these forever on the corner of their lot.
I agree with Gus about the rear main. If you do try to install it while in the car you're looking at about a 90% fail rate after all that work. Either live with the puddles of oil or pull the motor and do it right. Even then you probably looking at 50/50 that it won't leak eventually.