It depends on what you expect out of the coupler. If you're planning to use it to retune you Hustler from one band to another without changing the coil, it won't work.
Yaesu makes the FT-40 auto coupler which plugs into the 857. It will tune a 102 inch whip from 40 through 10 meters, not problem. The efficiency isn't all that great, but you will make contacts.
If you want to do more, I suggest you got to my web site, look under Auto Couplers.