If you compare plans of the 18s and 19s, measuring all from the center of the cylinder saddle: The c18s first driver is 1 foot farther, the seccond is five inches closer to the saddle, the third is 1 inch farther than the 19s, and the same for the last driver as well. This gives the c18 a 10'-10" wheel base which is shorter than the 19s 11'-1" wheel base, but the 18s start 1 foot further back on the loco than the 19s. I belive the difference of the firebox is due to a longer boiler on the c18. The frame of the c18 is almost a foot longer as well.
Also, the main drivers on the 17s are the same distance as the 18s, but the wheel base on the 17 is the longest of the inside frame c's at 11'-9"
The main drivers on the baldwin c16s are 4" closer to the center of the saddle than the 19s, and 2" on the grants..
Casey
PS, If this is confusing, I am sorry. Ill shut up now.