Welcome to my world. With all the car and loco bashing that I do I'm always dealing with "not made for this" couplers and adaptations. 2R to 3R can be an adventure. Or easy.
First - by "too short" do you mean that the coupler head doesn't protrude at all (not good), or that the coupler head protrudes only a bit (this can be good for appearances and car gap)?
But - check on eBay for couplers that are longer. Sometimes a measurement is given. Check Lionel's parts on their website. They almost always give a measurement.
You may/will have to deal with the (coupler) mounting screw and the centering design. These do not appear to have the once-common "standard" (and well-engineered) centering design as can be found on many 3RO articulated couplers.
It can sometimes be easier to adapt other trucks to your cars with the measurements you desire, rather than re-engineer the couplers on the ones you have.