"The threaded throat is bolted on, but the 30162 will not fit, unfortunately"

Maybe an adapter plate could be made. Pictures and possibly dimensions of the internal rectangular port with the bolt hole lay out would be helpful. After we have this the same dimensions from the 30162 adapters would tell us if a simple plate could be used to adapt 30162 to your existing horns. But due to Murphys law, that would be much to simple.

Or due you mean that there is no adapter from the horn to the thread mount? That the end of the horn is threaded? If that is the case, making the adapter gets much more complicated.

Like I said, a picture would be very beneficial.

BillWojo