I'm replacing a sliding patio door with a french door. Unfortunately the Rough Opening of the existing frame is a bit too narrow (the height is fine). Apparently my current door (22+ years old) is/was a non-standard size; it measures 70 1/2" wide. The RO width is just about 71".

Additionally the existing wood framed unit has extruded aluminum extensions that provide an edge for siding to abutt to. Ergo from the outside the old unit (frame-to-frame) measures just 68" wide, while from the inside the unit frame dimension is 70 1/2".

I'm replacing this slider with a french door that needs a R.O. of 71 1/2" at a minimum, but 72" is recommended.

I think I have two (2) jobs as regards this.

1. I have to support the header, remove the jack studs, and trim both to get the basic rough opening size. No problems there (I think).

2. I have to cut back the vinyl siding (and foam insulation board). I'm just not sure how to do this. I'm guessing that once I pull the unit I'll need to cut the vinyl siding with a circular saw and somehow re-install the J channel top and sides. That's my dilemma, how is that done since J channel is usually installed FIRST then siding is applied. Here I'm cutting the siding so the new door can be installed, then installing the J-channel....

Do I drill a pilot hole in the siding so that I can slip the J-channel in place and hang it with a nail? Do I just use chalking to stick the J-channel to the sides of the door's frame and let the adhesive hold the J-channel in place?

So my guestions are ...

A. Can I use my plain old 7 1/4" circular saw or do I need to buy one of them new fancy 3 1/2" or 4 1/2" trim saws? (Either way I understand that I can't trim 100% of the way to the corner so I'll have to use a utility knife or something to get the exact corner cut.

B. I'm assuming I can rip 1/4" or 3/8" off each jack stud to a thickness of 1 1/4" or 1 1/8" and re-install the jack stud. Is this going to be OK?

C. What's the best way to re-install (or install new) J-channel once I cut the vinyl siding back, adhesive or some other method?

Thanks in advance! Dave