I don't have the S3, but the BN's diesel diagrams claim that the S2 and S4 both had an overall length of 45 ' 6", while the S1 was 45' 5 3/4". I would think that there would be at least 1/4" difference between locos in an order, but that's what they listed.
They don't give the wheelbase, but it seems odd if they were the same length, but had different truck spacing. I would think that Alco would use the same frame to save money. On the other hand, the Diesel Spotters Guide says that both the Blunt and AAR Switcher trucks had a 96" wheelbase, so any difference couldn't be the truck wheelbase.
I'd say kitbash one, and if it looks like right to you, run with it. If anyone turns up with a ruler and complains, tell them to build you a better one!