Now you have to look at BIOS modding on the video cards, Search google how to BIOS mod you type of video card. Warning you can brick your cards if you are not carefull
You can download this from here or this website, once downloaded run the compute mode program and change all cards to compute, then restart the computer. You will see the difference.
You have 2 options, either install windows 10 and run them in compute mode or follow the instructions on this page for compute mode in windows 7. I can confirm that you can get 20+ in windows 7 as I have 2 systems with this method with RX580 getting 31MH/s per card mining etherium.
Good luck.
I do not have any 90's that you are looking for but have alot of other fittings. All of these are Koolance fitting and 95% are new. I have a quick disconnect new and used and then a bunch of 3/8 to 3/8, 3/8 to 1/4 and so on. Let me know if you are interested. As for price we can talk in PM.