The heater blower cover in the e30 firewall must be cleared of all soundproofing. All threaded studs that extrude from it need to be ground down. We made two brackets in the top to support the plastic e36 wiring junction box, as shown below.
The second modification, shown in first picture, is to the inlet and outlet heater core aluminum tubes, next to the brake booster. They must be bent down and to the left of the car as shown. This move is tricky because aluminum tubes will break if treated roughly!