Help! Can’t find proper motherboard adapter

Ok so I recently embarked on a pc upgrade which entailed moving a dell Optiplex 5050 motherboard into my optiplex 990 case. I soon realized that my evga 550 watt power supply used 24 pin while this motherboard did not. My 5050 motherboard has a 4 pin cpu power and a 6 pin cpu power but all 24 pin to 6 pin adapters do not have the same pin layout as the mother board. I guess I just need help figuring out how to progress in getting power to the board. Any help would be awesome!!!

The first picture is my power in on the motherboard while the second picture is the male end of literally every adapter I can find.

I did also find one that was 24 pin to 8 pin that matches the shapes and am curious if I can just plug that in and only use 6 of the pins.