I use the same board with 5 gpu's type  r9 290 cards.

No shortening needed for a Asrock 970 extreme 4

I use two 16 x16 riser cables on the first en second 16x slots. The are working than at x8.

The last  16x slot can only be occupied with an 1x riser cable, because an 16 to 16x cable in this last slot  would block the pcie x1 slots from working.
the other x1 slots can now work with  x1 risers (see manual).

Install one by one starting with the first x16 x 16 cable than the other 16 x16 cable on the next 16 slot and than a x1 cable in the last 16 slot
Always easy continue to install in this hierarchy

It some time takes a lot of energy to customise a system.It is although strange  that the same type of board is not always working the same way.


Good luck