The bolster casting for the 3’7” arch bar truck was made for On3. It has not been modified to specifically fit the On30 wheel sets. So, when you put On30 wheel sets into our On3 bolsters, the brake shoes (hanging from the bolster casting) are off-set, in, from the wheels by 3 scale inches. You have to decide if this is OK with you, before you order these On30 trucks.
Another issue with the bolster casting is that the beams that support the brake beams rub against the inside of the On30 wheels (caused by the wheels being a scale 6” closer together than On3 wheels). The solution is to scrape away enough of the beam to clear the wheels. Use an Xacto #11 knife or similar perpendicular to the beam and scrape it (don’t cut it, you may cut too much).
We don’t have plans to make a new bolster for On30. Our die makers are all gone, and getting a mold made outside would be cost prohibitive.