If you use the factory '68-'72 BBC mounts, be aware that they situate the engine a little farther to the right than the SBC sits, probably to clear that pesky steering gear. They also used a different transmission crossmember with a matching offset for BBC 4-speed cars. The SBC crossmember will leave the tranny c0ckeyed, if it will bolt up at all.
The big hole in the bottom where you access the mount bolts on the SBC unit is oval, while the BBC one is more squared-off. If you can't dig up a used one, a restoration parts supplier can fix you up. If a Nova place can't, try a first-gen Camaro place.
Like Eric said, be sure your axle tubes are welded to the center section, and also that the spring perches are fully welded to the tubes. Some aren't.
Also, toss the T-bolts that hold the springs to the perches. Use U-bolts, two on each perch.