Russel, sorry, don't have technical data for you regarding torsion bar rates, however, if I was you I'd look at the torsion bar setup on an early 911 - something in the '67 to '73 period. It's a pretty basic and functional setup - lower wishbones with the torsion bars connected to the inboard end of the wishbones with an upright for the hub and brakes that is connected to a vertical damper. The top of the damper can be moved fore/aft or outboard/inboard to adjust caster and camber. Ride height is adjusted by rotating the torsion bar - there is a different number of splines on one end of the bar v. the other end so when you rotate the bar you get relatively small degrees of change in the position of the wishbone relative to the chassis. For large adjustment you can, of course, simply rotate one end one whole spline but that's pretty dramatic.
I would guess that the stock torsion bar is about right for your Mclaren given that you're probably going to want a little stiffer setup than a stock 911 front end.