I have a 4 post lift too and I have EVERYTHING and I mean EVERYTHING removed from the undercarriage. I have been stripping and grinding and cleaning the bottom a little everyday for a month. I also have had the axle, leaf springs and drive shaft sand blasted and painted. I have all new parts including the original match exhaust pipes, resonators, mufflers, heat shields, fuel lines, gas tank, brake lines, bolts, screws, clips, etc. So I have spend a bit of money. But I really like the this matching # 440 six pack Challenger so I want the bottom to look as good as the rest of the car.
But I'm still undecided about painting the bottom the final color. I think the dark grey would look good but I don't want to do the overspray thing even though like Alan said that it would be most original. I would just paint it black but then it would be difficult to see all the pain I've been though restoring the undercarriage....haha. I guess I'm vain. Maybe just go with dark gray or body color.
Still trying to decide.