Undo the speedometer clamp on the firewall down towards the accelerator pedal. This will allow the cable some slack and gives you room to reach behind the cluster to undo the cable at the speedo head.

Don't forget about the oil pressure line. The rheostat can also hit on the bottom of the binnacle. After the speedo cable, power plug and oil line are removed, tilt the top of the cluster towards you and lift in an upwards and forwards (towards the steering wheel) direction.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now