Youre right, box 6 (on the unique trucks also worn by SP stock 166) and D&RGW 5771 are both owned by the Eastern California Museum, the 5771 comes from Knotts Berry Farm, hence the steps going into the car.
As for the boxcars at Keeler, if I am not mistaken the one at the engine track is an old SG car, although I have never heard what it is/number. THe passenger car you refer to is probably 4, when I was in Keeler last it was dark and I couldnt find it. (warning to those poking around Keeler: Jim Saylor has said to me that those who live in Keeler are an odd bunch: will shoot first and ask questions later)