When the winder is attached to the camera, the base of the lens support is lowered by the height of the winder. The “extra rods” seen in the clear plastic bag in the second photo of post #1 are placed vertically between the front of the base and the front support which raises the cradle and rollers to compensate for the height of the winder.
As noted in post #4, trying to use the shorter lens support intended for the zoom lens won’t work with the 350 APO.
Each of these lens supports is designed so that the tripod socket on the bottom of the base is located at approximately the balance point of the camera and lens combination. That’s true of both models, long and short. Using the short support for a long lens—if it could be mounted—would likely give a “nose-heavy” combination.
The critical problem is: The support cradle that carries the rollers is designed to fit the smaller-diameter front section of the lens barrel (measures 79.86 mm diameter). By examining the photo of the short support alongside the millimeter scale in the 3rd photo of post #1, it appears that the rollers are about 100 mm forward of the large knurled attachment screw on the bracket (which fits the camera’s tripod socket).
I mounted my 350 APO with its dedicated lens support on an RZ67 body. With the lens fully retracted, the larger diameter rear barrel, aperture preset ring and so forth, are already farther forward than the cradle and rollers of the short support. In the retracted position, the forward edge of the preset ring is 152 mm forward of the camera’s tripod socket. The resulting interference between the rear portion of the barrel and the preset ring with the cradle and rollers of the short lens support makes mounting the 350 APO lens impossible.
In order to use the components of the short lens-support kit to properly support the 350 APO throughout its focusing range, you’d have to design and fabricate a custom base about the same length as that of the long unit. And the tripod socket on the bottom of the main beam should be positioned at the same place as the socket on the long support for proper balance (75 mm forward of the knurled mounting screw). You’d also have to allow for the 46 mm travel of the lens from its retracted to its extended positions.
The main beam is aluminum alloy and measures 10 mm x 30 mm x 223 mm. It would required significant machining to accept the various parts of the short support kit.
Waiting until you can find the appropriate kit is the best course. If you keep checking eBay, eventually the correct support may be offered for sale.