Best thing you can do is keep in contact with them and have them sign an updated agreed contract...I know you already tried the contract thing and it didn't work...but at least they know you are serious about getting your car this summer.
Frank would say from day one the contract should have stated that for every day they go past the contracted delivery date money comes off the final bill....and for every day you would hold up the project/final payment you pay them storage fees.
You don't want to have this ruin the build or friendship you hopefully have with the builder...but as they get tired and you get anxious things can get weird. You really just have to ride it out and keep everything civil...it's best for all involved.
Best of luck with this! And remember great things come to those that wait!!
