Joe Lupinski just brought a SD70MAC over, and we tried it out with the two 18000s with 4-1/4" of straight track between them, which creates the 7-3/8" spacing. It ran through with cars in tow with no problem. The 18000s with the 150 mm track that’s included actually create a 7-5/8" spacing, so we had to shorten it to 4-1/4" (108 mm).
The 7-3/8" module standard apparently comes form the spacing you get with two 16000s face to face with no straight track between them.
Anyway, looks like we’re in good shape with the 18000s.