Which sentence is grammatically correct?

The sentence "Gosh, I could have had a V-8!" is grammatically correct because it properly uses the modal verb "could" in conjunction with the perfect infinitive form "have had." This structure is typical for expressing hypothetical situations or regrets about past actions. In this context, it implies that the speaker did not have a V-8 in the past but is reflecting on that missed opportunity.

The phrase "could have" allows for the articulation of past possibilities or alternatives that were not realized, which fits the informal tone of the sentence beginning with "Gosh." The overall construction clearly conveys the sentiment of wishing things could have been different.

The other sentences do not follow standard grammatical rules. For instance, using "be ordering" sounds unnatural in this context, and "coulda" while colloquially used, lacks the formality required for standard grammar. Additionally, "should ordered" fails to match the correct verb tense and structure necessary for proper grammar, as it omits the appropriate form of the verb "to have" following a modal.