I have a 70 westy and I changed the seats to a pair with headrests that I got
at a junkyard. The drivers side fits with no mods. The passengers side has a
rotary slide under it so that the passenger seat can be rotated to look out
sideways. I removed this and had to tack weld the slide onto the seat base.
No big problem.  As to the upstairs bunk I cannot help. There is a minor
disadvantage to the headrest seats; you have to pull off the headrests to put
in the front transverse kiddie cot(useful temporary storage place).

Have fun
sam williams