Ive done 7 of these what i have found is use the 836 in the 789 box trim of the rear of the coupler box about quarter inch them to get it to the rite hieght you need to notch out the pilot just a little so the frt of the boxsits in that notch . the notch is the thichness if the kadee lid cover, with this method it also keeps the coupler box from turning keeps it dead straight. the first coversion i did with the 836 was to leave the lid off the 836 box and bolt it down and it came out dead on with out cutting , but they are very close to the loco and large curves are needed but it does work fine..
Nick..