Awhile back I thumbed through the Comp Cams catalog and found that the 961 springs had almost identical specs to the original 235s and the 910 springs were similial to what a 261 would've used.
Here's the specs I used for comparison:
http://www.canadianmilitarypattern.com/1960%20235-261%20Engine%20Manual.htm and
http://www.compcams.com/Base/pdf/ValveSpringChart.pdf
