Paging @Bebop you have a comrade...If I remember correctly the 05 up D60 axle tubes are 3/8". I can measure as confirm.
The Rhodes 9" kit has 1/4"" tubes.
So the total tube thickness should be almost 5/8"
Or if you are of the metric persuasion. 9.52mm + 6.35mm = 15.87mm.
I actually started using a metric metal scale to do layout work and it really does make the math easier.
Not sure why the US still holds onto the standard system.