I'm fairly confident that the bandwidth of the PCIe interface is not important for mining. I am currently using a PCIe x1 to x16 riser in each of my two miners and have not had any problems. The other cards are on x16 to x16 risers and all seems to be well. I have not done thorough testing though, and I have not been able to get GPUs running on both of the two x1 PCIe slots on each board.
In theory your board should work just fine, but I have no experience with it. Worth a try as you found it laying around!
I'm going to try using x1 to x16 molex risers and hopefully will be able to get them both to work.