For a stock chassis, the middle section of the front crossmember has to be angled to enable the spring to be mounted parallel to the axle. The caster angle (the rearward tilt of the axle kingpins) is generally set at between 5 and 9 degrees to enable the steering to self-centre, and stop the car wandering as it goes down the road.
The exact shape and construction of the crossmember is not that important as it is hidden behind the grille shell or splash apron. It obviously has to be strong enough to support the rest of the car on the front spring, tie the chassis rails together, and provide somewhere to mount the radiator. The original Ford one is a pressing that drops about 2 inches or so in the middle for the spring mount, and includes a deep vertical flange at the rear that the engine mounts to. Most of the ones we've seen have been well and truly stressed around the spring mount, and since we were building the rest of the chassis it made sense to use a new crossmember.
Simple aftermarket pressings are available for a reasonable price, and, as with the chassis rails, would have saved a lot of time and effort. But, yet again, we were struck by that overwhelming urge to spend hours in a freezing garage sawing, filing, and welding up bits of metal. And saving another 50 quid.
After drawing the crossmember on our CAD package, and calculating the correct angles and dimensions to make the centre slope back at about 6 degrees, we started cutting up more of our 100mm x 50mm box section. The two tapered side sections were each made from three pieces of this, while the centre was cut from some heavier-wall 60mm box. These bits were all TIG-welded together to loosely replicate the original we were copying, excluding the rear-most engine mounting flange. Instead of using U-bolts to hold the spring-clamp in place, we turned four bosses out of 3/4" steel round, each tapped 3/8"-24 UNF. A flat face was sawn and filed on each one, and then they were TIG welded to the centre of the crossmember.
The last piece to fit was a filler plate shaped to clear the threaded bosses, to complete the rear flange. Then it was time to clean up some of the external welds, and the crossmember was ready to install. We think the finished part was worth the effort, and once the ends are trimmed to size, should fit nicely between our chassis rails.
(2004 Update: having seen more well-used chassis around, one thing we would probably do now is to weld on a strip of flat steel - say 1" x 1/8" - flush with the lower edge of the front face, to double-up on the wall thickness for a bit of extra insurance)