you can get a fairly cheap set of digital calipers from eBay to measure your interface with. they're not perfect, or of particularly high quality but for small modelling work they are acceptable, certainly compared to the cost of a professional set. then model that up. Once you have that you can assembly your new design to that and check tolerances etc.

unfortunately, even then you're going to probably go through multiple iterations, that is the nature of design and engineering of prototypes.

I can't get over the cost of that lego track on shapeways.