I think that there was some evidence that pointed to Lykken. IIRC I saw somewhere that his own sister said she had seen the car on their property with the two girls "slumped over" inside. They also supposedly found Studebaker Lark hubcaps when they dug on the property.
You are right, though, a car would be difficult to dispose of on land. I thought that maybe by searching Google Earth it could be spotted abandoned in some woods somewhere, although I know it's a total long shot.
The whole thing about the boys strikes me as odd, though. If the boys were innocent, I don't understand how they became separated from the girls for long enough for this to happen - and what a coincidence that this Lykken character happened upon them in the few minutes that they could have been separated?
If the car still exists above ground it almost certainly would have been spotted by now. Hunters, particularly during deer and pheasant seasons, walk almost every square foot of wooded area. Even land that is posted "No Hunting, No Trespassing" will eventually be covered by someone who ignored or did not see the signs.
Abandoned cars are not unusual in tree rows and old over grown farm yards. Farmers will often will often park them in the trees so they are out of the way. But the Studebaker Larks were never as common as other cars and now are sought after by collectors. Someone would have noticed a 1960 Lark.
I believe that if the car had gone into the river either by accident or deliberately it is also likely that it would have turned up by now. Over the last 40 years the river has been very high but also very low. Things tend to turn up eventually.
The girls left the hospital at 9:30, which was a half hour after sunset. They were last seen near St Mary's Church which is on Highway 48 one mile east of Exit 31. That is about 15 miles from the hospital so it was rather dark by that time. They asked the boys for directions to the party at a gravel pit and started to follow the boys. However the boys missed their turn to the party and had to back track. By that time they lost sight of the girl's car.
There is a gravel pit directly across the road from the church however that pit looks rather new. And since it is across the road from the church I think it can be ruled out as the arty destination.
I need to make another try at getting into my old hard drive so I can access my research. As I recall Pamela had been acquainted with Lykken through church and both of the girls attended school with him. The Lykken farm is only about eight miles from the church, so it is very possible that he may have been in the area on his way to the party. I believe that it is quite plausible that if the girls had encountered Lykken after loosing sight of the boys they had been following, they may have be willing to follow him if he had offered to lead them to the party.
Lykken never became a suspect until decades after the girls vanished, and other than dragging the river, law enforcement did not conduct a very extensive search of the area. Therefore it is possible that he had plenty of time to dispose of the girls and vehicle. When LE did start searching the farm, more than 30 years after the girls had gone missing, they did target specific buildings and other areas on the farm. I do not know what leads may have given them reason to be interested in specific areas.