Reliable is absolutely what matters. I’ve never done a Sbox but have rallied in 140s and 240s. In addition to Ramrod and Ian’s lists,I’d replace whatever holds the exhaust up and add wires underneath as backup in case one fails. Old wire coat hangers do the job well. Also decent fuel filters: the bigger the better. 240s can have trouble with the hose in the tank. I’d replace it. Or is B21A a carby model?
Think about wiring up anything critical that might vibrate loose on corrugated roads, especially sump, gearbox and diff drain plugs.
I assume you won’t need a sump guard?
Don't be tempted to use higher profile tyres to raise the car: you’ll only reduce its acceleration further! If it needs raising (and it probably doesn’t), the ghetto way at the back is with spacer plates. You could probably do something similar at the front.
Every night spend some time going over the car and tightening everything up. You’ll be amazed what can shake loose. It’s far easier to do preventive maintenance than fix something broken.
Take 2 spare wheels!