I think what OP is saying is that his tax liability is less than $7,500, e.g. it is $6,000. He will be taking delivery early 2018 so that he can "get as close to $7,500" as possible. I.e. He will get $6,000 as opposed to $3,750 if he waits till end of 2018 for delivery.
He claims to understand how tax liability works, so I believe him.
His only question in this thread was how to not pay the $6,000 via withholdings throughout 2018, so that he doesn't have to wait till early 2019 to get his $6,000 back.
I feel like a lot of people here are completely misunderstanding the OP. I along with a few others gave the OP some good advice on how to achieve his objective. Advice that he can explore after talking to a tax professional.
And yes, the tax credit can be taken away at any time, so sometimes it is better to just wait for the refund.