One way I have done it is to make my patch panel overlap the origional by 1/4 to 1/2 inch, use self drillng self tapping tec screws here and there to locate it, start at one corner first with a thin cutoff wheel and then also my air powered hack saw I cut through both pieces a few inches and tack the patch in. I will use a seal pick or thin pocket screwdriver to pry up or down on the edges to get the fit I am happy with. It works for me gives a perfect gap to mig weld and saves a lot of time trimming the floor patch to fit perfectly only to realize when its about half welded in it is somehow now too small, cut out of square, too big, or......
I'm in the camp that it's ok to lap (with a small overlap) weld floors aslong as you a paint and seam seal both sides. all visible patch panel work should be but welded.
Lapping a panel has the potential to trap unpainted steel parts together with moisture. Using seam sealer will help but it is inviting rust.
You will not ever get a consistent answer on this one. Most floors rusted out because water sat on them for long periods of time, they rusted from the top (or inside) down. If the water is still getting in and allowed to sit there, it won't matter how the new pans were welded in. If the welds are seam sealed (at the top) and the metal is painted, and water is not let sit, the floors hold up well. Lap welding works well on floors and structure points. Use a little thought so the lap doesn't hold water on the back side. Save the time consuming butt welding process for outer body patch panels (if you feel the need). I seem seal the weld joint from the top, paint the exposed bare metal surface (use the proper painting process), put a sheet of plastic over the metal, and install the covering. If your really concerned, don't use jute backed carpet, it holds water for weeks after it gets wet. Gene
Gene, Thanks for the info and yes it's been a little confusing, but I went the butt weld path on this one. It came out good so I'll proceed and finish soon. Primer, seal, and dynomat. No carpet going in, going with the original rubber type full floor mat. Thanks again to all of you guys for all the help.
I like to butt weld and hammer up. A little more work trimming up etc however results speak for themselves.