Well, I'm going with the Corsa C Column on Chris G-S advice. Rather than redrill the front hubs to speed up the steering ratio (if it turns out to be too slow) I reckon the easy way is a Quafe quick rack / pinion which will give the Sierra rack 2.8 turns lock-to-lock.
From those who have fitted the Corsa column, a couple of questions:
I've seen it said the Rover stalks will fit but mine are 3-4mm too wide to slot into the Vauxhall column fittings. Did you guys just mill a bit off the Rover switches to skinny them up and get them to fit?
Second, the Rover ignition has some hefty cables and feeds power to a number of fuses in the fusebox. The Vauxhall ignition has wires thinner than a five pound note! I guess a couple of 70 amp relays to feed the fusebox with the Corsa ignition providing the relay energising current is the way to go? Any experience?