Rust Repair: Other jobs to do at the same time?

I have the back half of the floor in my xj ripped out because it was rusted pretty badly. I had some questions on this job and also wanted to know if I should do anything else will I am back there.

Floor Repair:
How far apart should I put the spot welds going to the frame?
What should I put on the frame rails to protect them from rust but can still weld through?
Since I am using 18g steel should I put the spot weld holes in the old (thinner) sheet metal such as the wheel wells?

Other jobs:
I plan on breaking off all of the shocks mount bolts and replacing the bolts and nuts.
What can I do to protect the inside of the frame that is not exposed from cutting off the rear floor (the part under the driver and passenger seats)?
What else?

Thanks for the help. This is my first big fabrication/welding job
I want this jeep to last a long time so I am trying to kill all of the rust in one step. I plan on por15ing the bottom(outside) of the new floor before I install it and then doing the inside once it is installed as well as the inside of the frame. I also plan on stripping all of the paint off the old front floor and por15ing that too.

Katarn

i used a grinder on all the spot welds (seems like there thousands) and around the edges if there is still good metal intacked i used the fence of my jigsaw as a guid and left a lip all the way around so when i put the new steel in it would have something to rest on. obviously u have droped the tank but be carefull when cutting that u don't cut into the frame rails/supports there are 4 pieces back there. (the two main frame rails and two more above the tank) what i did for the rust prob was one use 1/8" plate so even if it does rust it will outlast any other piece on the car. but i also cut out the piece to fit as good as i could painted everythign even the plate then layed the plate in and marked where i wanted to weld then pulled the plate out and used a grinder to clean off those spots so i could weld. i guess you could try using a 6010 rod i believe (if you have a stick) they supposedly weld thru rust and paint but do all you can to make it as clean as possible. once i welded it in i sprayed a few coats of undercoating and let it dry put up the tank and was good to go. the hardes part for me was getting the steel to fit perfect. a good way to make a pattern is to get some of that super thin plywood 1/8" i beleive and cut it into strips a few inches wide then bust out you glue gun and jigsaw and glue in the strips to form an outline of where your new steel will go.

How far apart should I put the spot welds going to the frame?
Every inch is pretty good, I generally follow the factory seam and use the same amount of welds and distance between. I like to use a 3/8" spot weld but you can also use the 5/16" spot weld instead

What should I put on the frame rails to protect them from rust but can still weld through?
Go to any Auto Body Supply Store and as for "Weld Thru Primer" Grind all of your surfaces you are going to spot weld with 36-50 grit roll-lok disc or something similiar. And spray a LIGHT coat of primer on the surface. Place your new piece in and make sure that there is NO gap between the new and existing pieces. I use screws to hold thing together tight and then weld it up. Then pull the screws and weld those holes shut as well.

Since I am using 18g steel should I put the spot weld holes in the old (thinner) sheet metal such as the wheel wells?
I would spot weld all around the new piece.

Get some automotive seam sealer (NOT SILICONE)and do all the seams when you are done, that will ward of any water leaking in and starting new rust holes.

Hope that helps
If you have any other questions about panel removal and installation PM me.

Thank you for all of the suggestions.
I have already pulled out the floor, but the gas tank is still in. I cut all around the floor and then rolled it up like a sardine can, popping the spot welds along the way.

I started to make the rear most panel today. It goes from the rear hatch latch area to the middle of the wheel wells. So far so go. The nibbler I used to cut out the panel was alittle slow but it did its job and put the new compressor through its paces.

Katarn

I wish I had the pictures I took developed and scanned.

When I did my rear I did pretty much what XJourney said but I painted inside the frame rails after the primer but left the flanges that you are going to weld to without paint, just the primer.

I put in a peice of 1/8 diamond plate in so now its one peice from the back of the rear seat to the latch and side to side. I also built a new gas tank filler cover out of it as mine was rusted up too.

Because of the plate I used I welded from underneath, the edge of the flange to the bottom of the plate.

Later on I'm thinking of adding another peice of plate to the back of the rear seat so when its folded down it will be a bit section of it for cargo.

Yes its a bit heavy but I figure its worth it for the stiffness. Since what we are welding in doesn't have all the ribs of the original sheet metal.