Karl, closest I can give you on the offset is that the centreline of the standard 4.5" TR3 steel wheel is 1" behind the wheel mount surface of the rear brake drum.
On a standard car the front track on steel wheels is 45".
I don't know if wire wheels had a different offset to keep the front track to 45", as there are no known dimensions for a car fitted with the spline adaptors.
Remember your spare wheel cavity only takes a 4.5" rim, so a puncture on a wider rim has to be carried for repair behind the seats or on a luggage rack.
Another (strange) thing to watch is that many cars had a body mounted offset to the chassis, so a wide wheel can rub on one side at the rear, usually the left side.
Cheers.
Viv.