The boot floor isn't load bearing - but the rear tubs are and anywhere within 30cm of a suspension mount will be too (not too clued up on RWD cars so i'm not 100% sure how far away the leaf spring seats are from the boot floor or whether any part to do with the axle is considered structural).

The idea of ducts in the side windows is perhaps the wrong terminology - if you use a scoop under the car to draw air up and across the vented floor (to which the radiator is positioned) the air is 'sucked' in from the ducts rather than forced in and over the radiator.

Theres a lot to it and there will be a fair amount of trial and error involved.