I don't quite understand what you want to do, but maybe this will help.
I have some Spectrum medium tenders. They have identical trucks. The drawbar
is a separate part that fits over the mounting post with the front truck, but it also has the bronze wires in it
to contact the fingers on the front truck.
The coupler on the back is body mounted.
I usually just cut the whole box off under the back, put a 1015 or 1023 on there (whatever I have handy), and shim it for the correct coupler height. I seem to recall that for some couplers, I had to file the underside of the chassis (as opposed to shim)
to *raise* the coupler to the correct height, but I don't have any I've done handy here at the moment.
You could make your own drawbar, but you'll have to do something about the contact wires - either copy the design of the
Bachmann wires, or just hard-wire up inside the tender.