It will take some time, so be patient, you will have to use the cutoff wheel at bottom of 1/4 to remove from sill.
There is not really enough room to drill out from the inside.
And again reattaching the new 1/4 to sill is difficult, hard to get your head and welding helmet down from the inside at the same time.
Have always wondered how the assembly sequence went in the factory.
Enjoy the rebuild.