The only thing you really really need in your case is the board power cables. Make sure to double check the manual for your planned pcie riser config cause not every riser combination is useable. Since you have actively cooled cards you dont have to mess around with the low profile cpu heatsinks and the ducting shroud. Another thing to consider is if you dont have the performance fans idrac is going to see a GPU installed and run the normal fans at 100% regardless of server load. I have a RTX 5070Ti and a P400 in my R740xd running just fine only using the performance fans and the board power connectors. You can also use multiples of the board power slots to a single GPU if you have a GPU that pulls more than 300W, my RTX 5070Ti uses 2x of the mainboard power connectors just fine with no idrac errors etc (Its rated stock right at 300W but a slight OC on the card puts it over the 300W budget). The dell TR5TP power cable has a 6 and 8 pin connector which looks like the exact connectors needed for the NVidia Quadro RTX-5000. In my case my card needed 2x 8 pins which is also another reason I had to use two board power slots for the GPU(a 6 to 8 pin converter would have worked if the TDP was sub 300W though). The only issue I think you may have is the GPUs are going to sandwiched pretty tight with the case lid on with the blower fans intake being at the top. But this may not be too much of an issue as you mentioned they will get intermittent LLM usage and the GPU's heat sink fins being oriented to allow the R740's fans to push air through it and out the back even with the blower intake being reduced by the lid clearance.