I researched this quite a bit.  The 1000w powersupply will not be able to fully power a 4090, due to its internal design.  Someone on reddit dissected them with pictures, the power output on these supplies are split into 3 separate rails (36amp / 27amp / 36amp).  

Both the 1000w and 750w supplies only have a maximum output of 432 watts over the gpu supply cables.  During gaming, a 4090 will pull 10-12watts max across the pci from the motherboard, which means in demanding games the rest of the 450watts is from the gpu cables from the powersupply, which will be pulling 438-440watts.  This will trip the 1000w supply and shut down your R13.   If possible, change your order to the 1350w supply which can do 864watts over the gpu cables.  If not, you will need to powerlimit the 4090 (about -10% to account for spikes) using something like msi-afterburner.

For size, the limiting item is the power connector on the motherboard.  A 4090fe will fit, and I also saw someone with the Gigabyte watercooled 4090 also fits.  Others might also, measure the distance on your motherboard to that white power connector.  You will also need to remove the metal psu shroud so that the intake fans have a sliver of space to grab some air.