Bob, my alum Thagard pan was slightly un-flat also... . As far as the continues weld, I'm not sure what you mean.
I used the orange gasket sealant that comes in the oem full gasket set, and I belive it will be ok as far as sealing. Mine pulled down flat once I tightened down all those 10mm bolts on the flange.
I really like your custom oil pick up. I honestly at this point, would feel more confident having that set up in my oil pan than the way I did mine.... only because it hasn't been tried out yet.
That pick up of yours will work fine I'm sure, plus its nice and suported.... props to your machinest.....