The prototype EMD FT's were attached by drawbars, actually (in A-B sets, I believe).
The drawbar's length can be determined best by trial and error; place your units
on the smallest diameter curve that you intend to use. Measure between your
mounting points on the trucks (where the couplers are now screwed in) when you have the units as close as you'd like without interference. Make a drawbar from brass or aluminum stock of sufficient thickness, and drill some holes on each end. If the truck design allows it, you can adjust the length as needed to accept some diaphragms and give
a good appearance.
This is a fiddle-with-it project, and requires compromise, but it's easy. I just did it
on an FA set that I upgraded (both units powered, etc) and I did not want to install
an 8-wire M/F plug for my tether. So, no uncouple, please!