It is looking great, the "Props" you made to hold up the skin look like a great way to go.
If you don't have a perfect fit for the butt welds you can use a "Backing" behind the weld. After you have "massaged" your metal with relief cuts or pie cuts or what ever you need to make the pillars line up perfect, you then can take a piece of the pillar you cut off and dice it up to stick it inside of the bottom pillar. This photo (I grabbed it off the net, I don't have a personal example) gives you an idea of what I am talking about.
A "Butt weld with backing" is a standard weld done in collision repair. It is in fact required buy many manufacturers in splicing any piece of their cars. I has however been dropped by many because of corrosion issues but in your hot rod, this is hardly anything to think about.
Anyway, your fit doesn't have to be perfect, and in fact you WANT a gap using this welding method because you direct your weld to the underlying piece and "bring" the weld up onto the adjacent metal.
The backing should be all the way to the end of the weld (unlike the Model A in the photo). You can take your time and do this fitting,bend and match the piece until it fits like a glove (this is VERY easy to do) then "Plug weld" it in as on the photo.
A few plug welds will hold in place until you get your roof back on then you can weld the "Butt weld" (with backing
). You still need to be careful, VERY careful not to weld to fast or you will end up with a warped mess. Going across that roof should take you ALL DAY! I am not joking, a little 1/4" weld on one side, go all the way to the other side of the roof for another then work on something else, a weld on a post or something to let that weld up on the roof cool NATURALLY. DO NOT COOL THE WELD with water or air or anything. Cooling SHRINKS the metal and cause all kinds of trouble. Just make very small welds and let them cool naturally while you are off doing something else. There is PLENTY to do on the truck right? You won't be bored waiting, right? I didn't think so.
Welding across that roof really has to be given a LOT of respect or you WILL end up with a warped mess with a LOT of filler. The metal is so thick it will hold it's self in place pretty good. If you make these small welds you and letting it cool you will be able to pull it off without a bunch of hammer and dolly work (though some will be needed of course) but if you weld it too fast or cool welds it will be VERY challenging.
So far, loooooooking gooooooood.