I think that I can help with the 1969 STP chassis identification. I was a Chrysler engineer in the STP-Plymouth engine program in 1969. I was responsible for the interfaces between the STP chassis and the Plymouth engine/transmission and the Ferguson transfer case.
Both the #20 and the #40 cars had STP built tubs and suspension that looked to me like direct copies of the Lotus 56, although Vince Granatelli insisted that they were original designs, not Lotus copies. They were definitely not Lotus built chassis. For Indianapolis car 20 had a Plymouth stock block engine and car 40 had a turbo Offy. Car 20 had two bodies, the "Super Wedge" and a smaller body that looks like a Lotus 56 at first glance. The "Super Wedge" was built in a small shop just down the street from the STP facility in Santa Monica. I don't know who designed it.
The Super Wedge was designed to fit the stock block Plymouth engine with stock iron heads. However after the body was designed we switch to Weslake designed aluminum heads. These heads were very similar id design to the Gurney-Weslake heads, but were to fit the Plymouth engine. Harry Weslake deigned, built and delivered these heads in less than 90 days, and they gave a power increase of over 40 HP. But they were too tall to fit under the Super Wedge body. Andy Granatelli was happy to exchange 40 HP for the trick body, but wanted the body on the car when it arrived at the Speedway. Andy knew that the body would generate a lot of positive PR. The engine with the Weslake heads arrived at the garage two days after the car. The engine was installed together with the smaller body and the Super Wedge body was put in the garage and never seen again. Art Pollard was the assigned driver for the #20 car and did all of the test driving. Unfortunately we had some lubrication problems and wiped out the crankshaft thrust bearing on three engines. With the questionable reliability of the Plymouth engine, Pollard switch to the #40 Offy powered car and qualified it. We withdrew the Plymouth engined car from this race. The whole program wasn't started until the January 20, so our time to design test and develop the engine was very limited.
The #40 car retried with a failure in the lube circuit of the Ferguson transfer case.
The #20 car with the Plymouth engine ran in eight races driven by Art Pollard, Sam Posey and Jim Malloy. Its best finish was third in the first heat at Kent, Washington.
For the final race of the year at Riverside car #40 also ran with a Plymouth engine, divan by George Follmer, with Sam Posey in #20.
Car #20 at Kent and both cars at Riverside ran automatic transmissions based on a Chrysler Torqueflite.
The Plymouth engine was also run in four races in Grant King's Gehardt chassis, #57, driven by Art Pollard. Art won the race at Dover with this car.
The original iron head version of the engine was installed by Grant King in two of his dirt cars driven by Greg Weld and Art Pollard. A Plymouth engine sat on the pole in all of the Champ Car dirt races that year