Looking for advice, BB headers vs. HP manifolds

3 is a huge pipe and there aint alot of room for them back there over the rear and outboard of the tank.

You aren't kidding. I had 3" dumping before the axle for a few years and decided to make some tailpipes for it. I ended up sticking with 3", just because, even though it wasn't needed. It took a bit of effort to snake it through the back end with a car that has been mini-tubbed. Next time, I would go 2.5" just for ease of install.