Why move the porch at all? It seems like it would be easier to just
climb under the porch and add your metal siding underneath the porch.

I was looking at this today and I'm starting to think that this is the
solution. It would be tough working under there, but a lot less
trouble than moving the porch. All digging would have to be done with
a hand spade, rather than a shovel, but I can live with that. But
there is a problem. The porch sits about 6" lower than the house
floor. I would not be able to nail my framework under the floor
joists, nor screw on the metal siding.

I haven't seen the situation, so I'm just guessing what the conditions are.
But didn't you say the deck was free standing and not attached to the
house? Is there a gap between the house and deck?

Climb under and take some measurements of the opening. Then preassemble a
frame (or smaller frames) out of pressure treated lumber. Slide the frame
(s) under the house and screw it to the bottom side of the house joists.
You could either attach your siding to the frame before you slide it under
the house, or attach the siding after you have the framework in place.

If you really need to attach the siding behind the deck, just drill a few
holes in the deck rim joist (or end joist depending on the joist
orientation). Then you can access the house through the holes to drive in
your screws. If it's too difficult to do from underneath, maybe you could
remove a deck board or two to gain access.

Anthony Watson
www.mountainsoftware.com
www.watsondiy.com