UPDATE!!
Finally the warm...well warmish weather has returned and I can get back to work on the pedal car. Over the Winter I went back to the drawing board for this and figured out how to fix the slipping chain, the syncrhonizing of the pedals, better overall weight distribution, and further structural support. All I needed to fix a lot of it was one more matching bike, problem is they are quite rare these days for this particular model. But right at the end of February I found a 3rd matching bike for sale and immediately bought it. Since I've had a bunch of free time due to the global crisis happening (Check local listings) I got to work on it and had the whole car up and running again in just a week. Changes I made from the previous design were:
-Added a central steel beam to add support and hold a third set of pedals.
-Linked all three sets of pedals to create one continuous moving pedal.
-Moved the 5th drive wheel closer to the seats to help give it more traction, better mounting, and shorten the drive chain.
-Used the back end of the frame to better hold the drive wheel instead of making one myself.
-Added additional cross supports for the front wheels to keep them in place.
-Added further frame support to keep it from wobbling or bouncing as much.
-Added further support to the back end, and a new solid full length bar for the back tires, making them adjustable and easier to change if needed.
Next things to do are add the passenger seat, work out some kinks with the middle pedals not wanting to rotate so easily, grind off the rust, and give it a nice new coat of black paint overall, tires included.