I cannot seem to find them, but I did find some for a 405, which is reportedly very similar. The numbers are about the same as for the 531, to the best of my recollection. All cfm are at 28 inches of water, but I don't know whether the rest of the details were consistent with what my shop used when doing work on my 530. Things like the bore tube, the intake, the exhaust, and the seals can all be important and create large variables. This is why you have to be careful comparing flow numbers when the tests were not done by the same person on the same bench. Before and after tests by the same person on the same bench are the best.
Here is data for a 405 head that seems to be reliable and comparable based on what I have seen for 160/398/405/530/531 over the years:
intake with lift at .1/.2/.3/.4/.5 inches flowed 51/102/146/161/169 cfm, respectively
exhaust with lift at .1/.2/.3/.4/.5 inches flowed 39/78/100/108/114 cfm, respectively
The 530 intake will more or less top out in the .350 range in stock form. The 531 keeps flowing like this 405. On the exhaust, both the 530 and 531 crept up in flow right to the end, like this 405.
With my current 530 head with moderate porting and 40/45 mm exhaust/intake valves, the flow at 28 inches of water was
intake with lift at .1/.2/.3/.4/.5 inches flowed 54/113/156/164/168 cfm, respectively
exhaust with lift at .1/.2/.3/.4/.5 inches flowed 48/83/107/125/135 cfm, respectively
Again, what is most important is not these numbers I have supplied, but what your head flows now based on your testing methods and what it flows afterwards. My gains were generally in the 10% to 20% range. The other important thing to remember is that flow numbers do not necessarily translate into driveability. You could hog out ports, lose velocity, get great flow numbers, and create an engine that is really inefficient in daily driving and that runs poorly everywhere but in a narrow, elevated rpm range. Numbers are not everything. There is a science to it, which is why good shops get $75 per hour and say they need to spend at least a day on your head slicing up old heads, mocking up ports with clay, doing numerous flow bench tests, and so forth. A "fluff and buff" that is pretty much light clean up work without fitting larger valves will help a little and is much cheaper. You could probably do this yourself without much risk
In my view, the multi angle valve job is the best bang for the buck, and costs not much more than a regular valve job, yet might get you 10+ cfm at some lifts on the intake. But before pulling your head and spending any money on it, try a performance cam -- it will make much better use of what you have (but will trade low end for top end).