FYI, empty gas tanks are more dangerous than full ones.  Liquid gasoline doesn't burn; gasoline vapor on the other hand is extremely flammable, and a potential explosion hazard.  Gasoline vapor has to be mixed with air to burn, like the air/fuel mixture entering an engine.  So a nearly empty tank, which has a lot of air in it, is much more dangerous than a full tank, with very little air in it.  A combustible air/fuel mixture is mostly air (ideally 14.7 parts air for every part fuel). 

In this case though I agree with MS, its not an issue, because you're working far enough away from it.  Just be mindful of the fuel line zigzagging across the undercarriage on the right side of the car.  Not for anything dangerous, just to avoid cutting it inadvertently. 

Your best bet is to cut the bulk of the pans out first, but leave maybe 2" around the frame rails and at the edges.  This allows you to fine tune those cuts and drill out the spot welds without having the weight of the pans hanging on it while you do.

I'll also add that this is a great time to install a set of subframe connectors to stiffen up the chassis.  This improves everything from handling to keeping the car squeak free.  Ford should have done this from the factory, yet here we are 52 years later and Mustangs still roll off the line without them. 

I was 14 when I bought my first Mustang and started working on it too.  Good luck, and let us know how we can help!

Another thing I have learned while working on floorpans:
If you leave the windshield in place, and sparks from a cutoff wheel or grinder WILL hit the windshield and burn a bunch of little pieces melted into the glass. They are permanent and will rust!
Place cardboard on the inside of the windows before doing any cutting or grinding.

Anotger tip. I find that drilling spot welds is a total waste of my time. Sonce the floor panel is being cut out anyway, i just use a cutoff wheel to grind through the floor at each spotweld, stopping before I get to the rocker panel or piece that is to remain. Then an air chisel will remove the floor panel easily. Even if you drill the welds with a spotweld cutter, you still have to grind the welds flat, so this skips the whole drilling process. A pneumatic flanger/pinch tool makes drilling plug weld holes unnecessary. Simply place the tool along the edge of the panel and hit the trigger to instantly punch a 1/4" hole.
Search floor pan installation on this forum to see how I did some a few years ago.

Greg B wrote:

Also NEVER use brake cleaner on ANYTHING that you are going to weld.  

A little more info on this which is important. Non chlorinated brake clean is flammable, which is self explanatory why its dangerous. The chlorinated brake clean which is not flammable is the one that really matters when welding. The high heat from welding releases the chlorine from the residue and creates a nerve agent, which can have serious short and long term effects. Here is a link for anyone that wants to learn some more about it or a quick google search will do the same. https://envirofluid.com/info-library/toxins/tetrachloroethylene-brake-cleaner

 

TKOPerformance wrote:

Worth noting that galvanized metal also produces toxic fumes when welded.  Its worth picking up a cheap respirator to keep from breathing that stuff in.  When you weld, grind, or cut through it you are releasing those fumes. 

The fumes are primarily vaporized zinc, which in this case have to be treated as a heavy metal such as arsenic or lead. This requires as least a HEPA filter such as a 3M N100 (a P100 is better). The N100's are $9-10, so not too expensive, but they should be disposed after the job because there is no way to determine breakthrough. Also, considerable care should be exercised fitting the mask to prevent leakby. Anyone that has sprayed lacquer with a filter mask is familiar with the paint streaks on either side of the nose where the filter does not fit properly. No matter what type of filter/mask you use it must fit your face and seal. An old school technique is to fit the mask to your face as good as you can, then apply a heavy coat of petroleum jelly to the rim of the mask to seal any remaining gaps. Just pretend you are Burt Reynolds.

Q66 wrote:

Thank you to everyone who responded. The car is almost ready to have floors welded in, but we want to make sure we know all the last minute things are done. Our main concern is the over lap of the new and original metal, and if there are any problems with having more overlap for structural support. We have seen that most people have around an inch or so of overlap, but the metal seems to be clean and strong down for around another inch and a half. If we can we would like to be able to over lap as much as possible to make sure the floors have a strong structure.

A lot of cars have the replacement panels butt welded to the remaining sections and never have a problem.  I believe a 1" overlap with rosette welds every couple of inches well be plenty strong.  Also, be sure you adequately seam seal the overlap under and inside.  That overlap is a very good hiding place for dirt which will absorb moisture and cause rust very quickly.

Great to hear that the project is coming along.  You and your Dad are very lucky guys.  Hope we get to see it some day.  And, don't be shy about posting pix...we love 'em.

BB
 

This has been very helpful, but there are places were the panel will be over lapping up to 2-2.5 inches. Should i be concerned and trim the panel down to a one inch overlap all the way around, or is it okay to leave the overlaps as is in those places.

I would not exceed a 1" overlap.  A larger inaccessible area is just asking for trouble IMO.  Treating, sealing, etc. are all good, but there's also something to be said for reducing the surface area as much as possible.