I have never heard of any capture card being able to do a Standads Conversion on the fly…post on the Blackmagic forum to be sure of that one.
I can attest to the fact that the Nattress plugin does a wonderful job…..not Snell and Wilcox level, but much less expensive. He has a free trial version, so you can test it on your footage.
Compressor 2 also works well, but takes much much longer.