I'm not an expert on 34 or 35 pickups but I didn't think that chassis was that much different from 34 to 35? I can't see shortening the frame 7 inches. Someone school me here please.
As far as assembling the bed. I would imagine it would be pretty straight forward, depends on your skill set I guess. I bought my 39 bed from Mac Hils but it was assembled. Did you call Mac Hils and ask if they could provide you with some assembly instructions? I found them to be very nice people.
Perhaps someone that has a bed removed currently could take some photos and post them here for you?