Marine Grade Plywood?
"Has sailing tech changed so much in the last 60 years or were these just crappy boats to begin with?"
At some level the answer is ''yes'' to both of these. Hull forms and rigs and evolved dramatically in ways that improve seaworthiness, ease of handling, motion comfort, and performance. BUT beyond that, these were really poor designs for their day.
The better D.I.Y. designs of that era came from magazines like ''Rudder'' and ''Field and Stream''. These magazines published designs ranging in quality from amazingly good to pretty bizarre. Many of the designs in these magazines came from the boards of designers of the caliber of Atkins, Crocker, John Hanna, Winslow Warner, Fredrick Geiger, Clinton Crane, Starling Burgess, and L.F.Herreshoff.
But most of the designs on the Svensen site are ''public domain'' designs that were prepared by unknown designers. The price of drawings are cheap compared to the hard costs of building a boat. It really makes sense to pick a good design even if you have to pay for it.
That said there are a bunch of good designs out there for free or nearly free it is just a matter of sorting them out.