Replace a Front Frame Rail - How Much Trouble?

Hi,
I've posted recently about some rust damage in the driver's side front frame rail of my 1968 Dart GT. I've been going back and forth about trying to repair the frame rail or replace it. Now I'm back to considering just replacing it but I've never done anything that invasive on a unibody car. I've posted two pictures, one showing a nice clean A-body driver's frame rail and one showing my wasted, rusted out frame rail. That hole is where I put a red circle on the clean rail. The damage goes right up to where the k-frame mounts to the rail so it's a difficult place to patch to say the least. I think it's a pretty poor design because the frame rail sort of forms a pool right below the upper shock mount so that dirt, leaves, and water can sit there and rust out the top of the rail. Mine obviously rusted the top of the rail and then finally rusted through the side and bottom of the rail. In the process of rotting out, the rust also took out the lower control arm bumper bracket, there's nothing left of that but rusty scraps.

The question is, based on this picture, should I consider trying to patch that area or bite the bullet and figure out how to replace the whole rail. One of the guys on this forum is a few hours from where I live and has a Barracuda shell with a good frame rail with the shock tower that he offered to sell me. My HOA would **** if I started cutting it up in my yard so I've found a place I can take the shell and remove the good rail. I'm just wondering if I'll be able to get the rotten one out of my car and put the good one back in place so that everything lines up so that I can move forward with my project. I was wondering if I could make some sort of jig to line up the tops of the shock towers before I remove the old one that I could use to help line up the new one. I think the K-frame will also be helpful to line things back up before final welding. I value any thoughts, opinions, and advice from some of you A-body experts. Just for reference, I've also uploaded a picture of the car. It's not great but it's not a piece of crap either, very worth fixing in my opinion.
I know I can't say it enough, this is a great place for people wanting to get their old Mopar A-bodies back on the road, thanks in advance for any input you might have.
IMG_9951.JPG Inked003-mopar-abody-rust-repair-frame-shock-tower-inner-fender_LI.jpg IMG_8498.jpg