Running a Mesa PCI card through a PCIe to PCI adaptor?

My mill uses a Jetway JNF9C motherboard, that I am starting to have problems with. This board has a n old-school PCI slot, in which I have a 5i23 card. 

I don't seem to be able to find a PCI slot mini-ITX board at a sensible price, so I thought I would try a PCIe-PCI adaptor. 
Specifically this one:  www.amazon.co.uk/dp/B0CHM91KJB

It almost seems to work, in that the 5i23 appears to be detected. But then fails with "/DONE not cleared...." (I can probably find a way to paste the exact output, but it's not straightforward as the motherboard is not on the network) 

Is there any point persisting with this, or should I just give up and buy a 6i24 to replace the 5i23? 
If so, which 6i24 do I need to run the somewhat special firmware (15 or 25)? (firmware=hm2/5i23/harrison.bit) Which is compatible with 7i49 and 7i39 boards (though I could drop the 7i49s as they are currently not in use) 

One issue is that I don't know which way round the 5i23 should be in the PCI slot, it's keyed to go both ways round.... (But if it is being detected then I guess it's probably right?)