We need something to keep the back of the truck off the ground, and that's the job of the rear axle. We haven't decided what axle to use yet (translation: we haven't found one cheap enough), but whatever we use, we are going to hang it from a '40 Ford rear spring.
A stock Model A rear spring sits over the axle, and has a high arch to clear the centre-section. This looks good on things like track-T's, where the rear is all exposed, but on closed cars or pickups the high rear crossmember imposes on interior or bed space. That's OK on an original like in the photo, but is a particular problem if the body is channeled over the chassis by much.
The spring perches on top of the axle also reduce clearance under the chassis, so limit suspension travel. When Ford introduced the Model B in 1932 they had moved the spring and crossmember back behind the axle to fix this problem. However, '32 rear springs are designed with a curve (viewed from above), to bring the ends in closer to the back of the axle, while the middle still clears the differential housing. These must have been more expensive to make than 'straight' springs, and would also have probably put a torsional load on the crossmember - whether this ever caused any problems though, we couldn't tell you.
Eventually Ford must have worked out that they could make the spring perches strong enough to reach a few inches behind the axle, enabling them to use a normal straight spring. The rear spring from a '40 Ford seems to be the most common substitute, so that's what we're using on the 'stock-style' chassis.
All we need to mount it is a simple 50mm square box-section crossmember, notched and bent before welding to give a 2" step. This will give a bit of extra clearance without getting in the way like an original one would have done. The spring mounting plate and its clamp are made from 3/8" thick bright steel flat. We bought a ten-foot length of 4" x 3/8" flat to make our suspension brackets, brake caliper mounts, and other stuff. The £40 it cost is probably worth it for the exercise involved in hacksawing it up! The wider bracket is welded to the crossmember, and has clearance holes in it, while the narrower one is the clamp plate, and is threaded.
After drilling a 1/2" hole in the tube for the spring location bolt, to match the one in the mounting plate, the crossmember and plate were fully TIG-welded. When this had cooled down, the cleaned-up spring was clamped on for a trial fit. We don't know what shocks we're using yet, so mounting brackets will be added at a later date. We will also remove spring leaves as necessary to achieve the correct ride height, so the spring should have about a four-inch arch when loaded.