So... picked up the Allard chassis last week and got stuck in..
The chassis is quite light and strengthened by a cruciform cross section where the gearbox sits.
For each of the strengthening sections I make a cardboard pattern from picture frame card. It's easy to 'form' and bend into the exact shape which I then use to make the steel pieces from.
The steel I have has an orange coating making it ideal for score marking to bend in the right place. Next stop, several days of welding to finish the chassis, then we are onto rust prevention and painting!