Propilot, my firewall was in excellent shape, so I had it sandblasted & coated with a heavy duty primer. The tub just bolted right on, but with all aftermarket items, a little coaxing and working was necessary, but surprisingly very little. Once I had the tub and firewall back on the frame, it was excellent motivation to get my ass moving to finish it off. I had the underside of both firewall (original) and tub coated with underbody coating (don't remember the brand, had a local guy do the blasting, painting, and coating). I'd do the same thing over again, and likely will when my fiancee gets her 40!
My dad's hunting buddy got a used steel tub from Arizona, which worked out well for him as well. So either route, you'll end up with something good. If you do go steel, then anti-rust the hell out of it!
Either way, have fun with it and enjoy it! Whatever
you are comfortable with or want, go for it. All we require is pics, and lots of 'em
Brian