That turned out alright...

[Lord Popgun]

This is what I found. Maybe someone else can do better.

At or near the front—1 amber on each side of the vertical centerline at the same height, and having a minimum horizontal separation distance (centerline of lamps) of 16 inches. Minimum edge to edge separation distance between lamp and headlamp is 4 inchesAt or near the rear—1 red or amber on each side of the vertical centerline, at the same height and having a minimum horizontal separation distance (centerline to centerline of lamps) of 9 inches. Minimum edge to edge separation distance between lamp and tail or stop lamp is 4-inches, when a single stop and taillamp is installed on the vertical centerline and the turn signal lamps are red.

Source: https://www.law.cornell.edu/cfr/text/49/571.108

Which is 49 CFR 571.108 Is way down in the document past some semi-trailer diagrams.