Phil, let it hang off the mid plate. I have never run a rear mount in my builds with either stock or aftermarket cases and have had zero problems. Although the chassis flex in a dragster or altered should be just about non existent in that area why mount a rigid engine/transmission package at three points and take the chance, especial;ly with a stock case. Not having a cross member at the back of the trans also makes it easier to pull the box--six bellhousing bolts and the shift linkage and it can go out the bottom of the car. Your idea of mounting from the pan rail (I presume that it is a shorty glide with no rear mount) is also overly complex and, again, more work when you want to pull the trans. Vaught's car is 42 out with a shorty making the driveshaft about a foot long and we have had zero problems in almost 10 years of running the car.
Roo