Ok, BlackGMC and 351w_swap, you got me thinking with the issue of running the exhaust out the back with your crossmembers in the way. I came up with an idea I'm going to use. Here's a picture of a quick layout I did in AutoCad. The center triangular opening is large enough for the driveshaft & 2 exhaust pipes. The small oval openings will be for fuel/brake lines and battery cables. I'll weld it up out of 11 ga. sheet metal.
click the thumbnail, there is an image there.