The following differential diagnoses are provided for the given scenarios:

Ventricular Tachycardia (VT): In the context of a patient with a history of old anterior MI presenting with tachycardia and a wide complex tachycardia on ECG, along with irregular cannon A waves, VT is the most likely diagnosis. This is because VT is a common complication in patients with prior MI due to scar formation in the ventricle, which can act as a focus for arrhythmias.
Hypertension: For the 60-year-old man who underwent a stress ECG test and stopped due to tiredness with a high heart rate and very high blood pressure, hypertension is a significant finding. The high blood pressure, especially in the context of exertion, suggests that this patient may have uncontrolled or severe hypertension.
Torsades de Pointes: Given the ECG description, Torsades de Pointes is a specific type of abnormal heart rhythm that can be identified on an ECG. It is characterized by a rapid, irregular heartbeat that can be life-threatening if not treated promptly.

Supraventricular Tachycardia (SVT) with Aberrancy: Although less likely than VT given the patient's history and the description of the wide complex tachycardia, SVT with aberrancy (where the tachycardia originates above the ventricles but is conducted to the ventricles in an abnormal way) could still be a consideration.
Atrial Fibrillation with Rapid Ventricular Response: This could also present with a rapid and irregular heart rhythm, although the ECG description and clinical context make VT more likely.
Severe Hypertension: In the context of the stress test, severe hypertension is a significant concern and requires immediate attention to prevent potential complications such as hypertensive crisis.

Cardiac Tamponade: Although less likely, cardiac tamponade could present with tachycardia and hypotension (not explicitly mentioned but could be a concern in the context of an old MI and new arrhythmia). It's a life-threatening condition that requires immediate diagnosis and treatment.
Pulmonary Embolism: This is another critical diagnosis not to miss, as it can present with tachycardia and could be a cause of sudden onset of symptoms. However, it's less directly related to the ECG findings described.
Aortic Dissection: Especially in the context of severe hypertension, aortic dissection is a potentially lethal condition that must be considered, particularly if there are symptoms suggestive of this diagnosis, such as severe chest or back pain.

Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC): A rare condition that affects the heart muscle and can lead to life-threatening arrhythmias. It's less likely given the patient's history of MI but could be considered in the differential for ventricular arrhythmias.
Brugada Syndrome: A genetic disorder that can lead to abnormal ECG findings and increase the risk of dangerous arrhythmias. It's rare and less likely given the clinical context provided.

Acute Myocarditis: This is a significant contraindication for a stress test because the heart muscle is inflamed, and exertion could worsen the condition or lead to dangerous arrhythmias.
Other options like severe asymptomatic aortic stenosis, asymptomatic MI 2 weeks ago, and AAA >5.5 cm asymptomatic are also important considerations but may not be absolute contraindications depending on the specific clinical context and guidelines followed.