Even though the cab connections MAY be the same I would just do a complete swap out of the Proto board plugs etc. in to the Bachmann tender. I would like to believe that sort of wiring would be standardized too just like decoder wiring but I guess the only way you would know for sure is to place the two boards with connectors side by side and ring the connections out with a meter.Swapping the boards sound a heck of a lot simpler.
Allegheny2-6-6-6 They did make a long tender version of that Y3 not sure if you can buy it from walthers or not but it may be worth a shot.
They did make a long tender version of that Y3 not sure if you can buy it from walthers or not but it may be worth a shot.