Tom,
I see now what I was missing, and now it all makes sense.
You have several options on the design. I'm a big fan of keeping things simple, inexpensive and perminant.
After you strip off your shingls and 1X4, I'd install 2X6 rafters in place, one at a time. Use standard pine, nothing special. The facia should be something that will resist rot. You mentioned cedar or PT earlier. You can also use hardiboard, wich would be the very best in my opinion. There is no need or reason to block the rafters.
Spacing is going to be based on what you decide to use for insulation. Jim showed you some pictures of solid foam that would probably be what your looking for. I'd use fiberglass and put the rafters 24" OC.
After the insulation is in, you install your perlins. 1X4's work fine and can be spacee 48" OC.
This is were Chris suggests using the felt paper. You don't have a smooth surface for it over the perlins, so I'm not sure how well it will work. If you really want to use felt, then to do it right, you'd need sheathing. Installing this will probably be the hardest part of the job. Getting those sheets up there is a royal pain, it's extremely expensive and it doesn't do anything for a metal roof. It's your call.
If you do install sheathing, you might want to install shingles again. There are allot of different styles in composit ones that are very attractive. Shop around and do some searches online. It's a good idea to compare all your options.
If your still going to install a metal roof. I'd skip the felt and install the metal directly onto the perlins.
What is your timeframe? I'm just starting the dirtwork on a workshop I'll be starting on in a few months. I'll be posting pictures on it, which should clarify the basics of a metal roof.
Eddie