This is kind of tough for two reasons . One, you lost the key piece and two, you went through the trouble of stripping the old stuff and now need a new piece that if you strip that one too, may look a bit different than the old pieces.
This is where the powdercoating option will take care of that issue. Untreated aluminium will start to corrode almost immediately .
If you removed the old ones yourself then you should have an idea how to reinstall them, but like George said , they never seem to go right back on exactly as they came off. That six inch spacing seems to morph when you remove rub rails and trust me , the predrilled 6'' holes on new strips never line up.
I hate messing with rub rails almost as much as I hate installing windscreens !
All that said though , when installing new stuff from scratch I put the bow and transom sections on first and then do the side ones. The side ones are easier to mess with in terms of trimming for length, etc .
Ideally , you really want a new 12' section. The problem with having two pieces with the joint in the centre as George mentioned , is that it's impossible to get the nice curve on the rail. I made this error when I did my first partial resto on my boat back in the later 80's and had to live with the discombobulated look for 20 years.
oh , I'm talking about working in S/S here btw ..
Just because something's old doesn't mean you throw it away !