[lelix]

I would do it in sections.
Since you are also replacing the pan/supports, I would start from the floor up and first establish a perfectly level and symmetrical floor pan with the suspension pick ups(borrow a jig if you have to). Then, locate and true the front latch panel, and see how well it lines up with the previous work(more than likely it wont). and only then graft the inner fender pieces onto the floor and latch panel. For that I would make a locating jig as well, based on known dimensions(fender holes, etc). then, cut your existing inner fenders out(one at a time), make a rough cut on the donor piece an inch or two bigger than the piece you cut out, locate it on the jig you made, then mark for the exact cut. This way you will not compound problems on a possibly already tweaked/misaligned front end.
A small die or angle grinder with the thinnest wheel you can find will give you a nice cut.

Hope this helps