I found it easier to solder the two 40 pin connectors and then work on the component side of the board. Everything is through hole so it's easiest to just work up by height until assembly is complete.

The jumpers do the following

JP1: determines if A8-A15 should always be driven across the bus or only when the port matches.

JP2: fit this jumper to drive the TX from the secondary bus onto the primary.

JP3: fit this jumper to drive the RX from the primary onto the secondary.

If you really need 36 slots you should be able to solder the two 40pin connectors the other way up and install an inverted board between the front of the primary and the back of the tertiary. It will need a different I/O address to the secondary.

The board does not address power consumption power is simply carried across the bus. It might help to also wire the backplanes together directly using the screw terminals many have I don't know.

If you need a really long case hold a 6x4 index card up in front of your system and it should fit well within the profile. In the UK at least ebay is full of old index card drawers and holders - just be sure that it's a 6x4 not 5x3 size.