The issue here is the type of failure that's likely to occur when one of the struts or rails splits at a bolt hole. To expand on RHM's point, once one joint fails, other failures are likely to follow rapidly and dump your load onto a wheel. Unlike metal, wood has no plasticity to speak of, so deformation past its elastic limit makes it break rather than bend. This is what will happen if any part of the rack comes undone with a load on it. Also, over the course of a 75 mile ride you should plan to check the fasteners several times, as the load and shaking may cause them to compress the wood and loosen.
One reason I became disenchanted with the Pletscher decades ago was its lateral stability. The rivets holding the struts to the platform just weren't up to that task.
I think the problem with Pletscher racks and lateral stability has more to do with how they were secured to the chain stays. If you tightened up the plate-style mount with the flat surface even against the stays, it had plenty of lateral stiffness. However, you risked squashing the frame tubes (ugh!). The strap-type mounts were useless for any sort of load and did almost as much cosmetic damage as the plates. I made a bracket for a Pletscher out of an old MAFAC Racer bridge piece bolted to the rear caliper which works just fine.
