Ok maybe the later ones do have a larger spindle bore and arn't actually called a d61 that I'm not 100% on but even if it does and you want to upgrade to bigger axles all your trying to do is accomplish what a stock d70 is anyways. I'm just saying why waste time and money on an axle that isn't as good and you'll never gain the strength in gears as the d70 is just much bigger for more contact area and shear size.
Just giving him another option

Question though would those ford van ones be wider than our year trucks original axles. I know I once looked at a Dana 80 and it was at least 3"s wider. My 44's are already 18.5"s wide I didn't need them to stick out any more plus lockers/gears were brutal expensive with no a major strength increase.