I ripped my heater cores and hoses out and then just looped the lines back in on each other right behind the driver's heater core. It's not elegant, but it worked and I still have somewhere to tap into for hydronic baseboard heat someday.
My interior bulkheads are screwed in on the top and the bottom with those little galvanized L-brackets. I think I used the 3 inches version. Each stud is held by 4, one on each side top and bottom, with the exception of the end stud that only has the inside ones so they weren't sticking out. I also tied them together with stringers that are toe-nailed in so the walls almost look like an "H" on the interior. I filled them with some 1 inch white rigid foam insulation I just had laying around so that they wouldn't be boomy and echoing and then sheet them with luan. They were plenty strong before the luan, but it added a lot of rigidity when I did that so I prefer to think of my walls as being of monocoque construction.