phew... nice.
Well, using this technique is the only correct way to go. Professionally speaking. My own TA had the quarters replaced before I got a hold of the car, and it was done the easy way with overlap and spotwelds. This technique will hold up fine, BUT the overlap is SURE to develop corrosion unless you really really soak it with rustprotection.
I guess I dont have to advise you, but here comes some: when "butt-welding" ( hehe ) never lay the weld next to each other going in one direction. Much better to go at it one weld at a time and make the welds with at least 5" between them. Welder in one hand and airgun in the other to cool each weld instantly. This was you keep the metal as cool as possible. Not the biggest issue when welding floor pans, but if you want a quarterpanel to be welded in as smooth and flush as possible, so you dont have to straighten it out with 4 pounds of bondo afterwards, you do the welds the cool way :-)
if I was living nearby, I would gladly come by and give you advise!
Say, how did the T-tops install go btw? you know, the deal where I sent you those measurements...