Hi Hugh. Welcome
I think the important dimension is the wheelbase. The US specs use 66" which is 1676mm in real money.
Do you have your front springs & axle sorted out yet? This will largely dictate where in the chassis your front axle sits. I would be inclined to get the front laid out and then measure back to determine the location of the rear axle.
Front springs are a bit of a challenge to locate here in Aus, specifically the buggy cart springs commonly used in the US. So you may end up using something different, which depending on how long they are might push your front axle location forwards or back from dimensions shown in the Stevensons plans...
I'm guessing from the dimensions in the plans that the US buggy cart springs are 550mm long, which should hopefully give you a point of reference.