Henry Ford's method of locating the beam front axles in his early cars consisted of a triangulated arrangement comprising the wishbone and a beam axle, with a transverse leaf spring. The spring was mounted above the axle on pre-'35 cars and in front on '36-48 models, when the I-beam gave way to independent front suspension. The whole arrangement would pivot about the ball at the rear of the wishbone and the center of the leaf spring, mounted to the front crossmember. It was crude yet offered a huge range of suspension travel, especially when combined with a similar system at the rear.
However, with the installation of non-stock motors and the advent of dropped axles, interference problems with oil pans, or the removal of the wishbone's rear mount-whether on the crossmember or the trans, a la Model A-meant an alternative was required. The hot rodder's solution was simple: Split the wishbone and mount the rear ends of the now two separate halves to the chassis 'rails. Of course, little consideration was given back in the day to bumpsteer, Ackermann angle, or suchlike, and the split wishbone worked on hot rods with limited suspension travel, pretty much defining the look of postwar hot rods.
This is a Model A wishbone and a complete frontend. It's way shorter than all the later 'bones at 39 inches, ball-to-perch centers, but uses the same wall thickness tubing. The mounts at the axle end are thinner but are forged and can be gusseted if required.
We dealt with early axles in the August issue, and touched on wishbones then, but the simple fact is if you want to run an unsplit wishbone, you'll need to source one from the same-year car as the axle. This is because the distance between the spring perch bosses on early Ford axles varies, and hence so does the measurement across the front of a wishbone. The '28-31 Model A axles measure 36 1/4 inches between bosses; '32-36 axles measure 36 1/21/2 inches; '37-41 axles are 38 1/21/2 inches; and '42-48 versions are the widest at 40 3/43/4 inches. Even if you intend to split your 'bones, you still need to be careful, as there are two perch-boss sizes. The '32-36 axles and their corresponding wishbones have yokes 2 inches across, while Model A and all the later 'bones measure 2 1/21/4 inches, though it's only a matter of adding a 1/4-inch spacer to run a later wishbone or Model A version on the '32-36 axle.
Then there's the length of the wishbones to consider. The Model A 'bones are short at 39 inches from the center of the ball to the center of the perch hole; '32 'bones are 47 1/21/2 inches; the '33-34 is 45 1/21/2 inches; the '37-41 is 44 1/2 inches; and the '42-48 measures 50 1/2 inches. Bear in mind that those '35-and-later have the spring in front of the axle, the extension for which will need removing if you intend to mount the spring above the axle in your application, and the '42-48 'bones have a step in them just behind the spring perch. Ideally, the pitman arm for the steering should be mounted directly above the rear mount of the split wishbone if you're contemplating a drag link style to eliminate bumpsteer as much as possible, which is why choosing a wishbone as close to the required length as possible is advisable. However, with the limited suspension travel on most hot rods, this isn't as major a concern as might be imagined. Heck, even stock Model As had a drag link roughly half the length of the wishbone.
Here's a '40 Ford wishbone and axle with the spring mounted in front of the axle. Ignore the strange rear mount that obviously served some purpose in the past. At 44 1/2 inches, these make great split 'bones. | 
Here's a Lincoln wishbone for comparison purposes. The axle isn't interchangeable with Ford spindles, as the KPI (kingpin inclination) angle is wrong and the kingpins are bigger, but the wishbone is usable. It's wider at the rear than a '40, which comes to a point, possibly allowing it to be used unsplit and still clear an oil pan. | 
The '42-48 wishbones have this kink a few inches behind the perch boss, making them easy to immediately identify at swap meets. |