A few misc questions, defrost vent, speedo led's, wiring, computers

[hrc200x]

The project: '75 4x4 frame w/dual tanks, '83 cab, '84 incab wiring harness (fuse block out to doors cargo light, etc), '88 clip from blazer (quad stacked headlights) '86 suburban heater and a/c wiring harness and fiberglass shrouding on firewall.

The questions: Why is there a large gap where the "Y" shaped defroster vent hooks up to the large box that the heater core mounts in? I've looked on 4 other 80s chevys and all the same way.

Sunday I took the wiring out of the '84 donor truck. On the back of the speedo there was a plastic thing with 2 or 3 wires going to it that was bolted with a very small bolt, had like a 5 or 5.5mm head on it, went in close to the speedo cable? What is this?

There is a light green and dark green wire that exit the cab close to the distributor I seem to remember these being for dual tanks. Will that wiring be compatable with a '75 selector valve?

The '83 cab had a spot to mount a computer up under the glove compartment, the '84 truck and '86 burb did not. Did they quit them in '83 or move them to another location?