cancer only grows in acidic bodies ... an alkaline body can prevent or cure cancer.
This is certainly over-simplistic, probably meaningless, almost certainly useless.
Average body pH
I suspect this is not a meaningful thing to attempt to measure. In terms of cancer, it may be no more meaningful than average body thickness, average body density or average body conductivity.
Our bodies are made up of many materials with varying pH:
Stomach: 2.0 ±1
Stomach with food: 4.5 ±0.5
Adipose: 7.0 ±0.1
Blood: 7.40 ±0.05
Small Intestine: 8 approx
Muscle tissue spaces: alkaline
Muscle fibre interior: acid
Urine: 4.6 - 8
Should we make an average by volume?, by mass? Should it be a weighted average? Should we measure and monitor the pH of hundreds or thousands of different tissues, fluids and other materials in our bodies?
Cancer and pH
Cancer is essentially uncontrolled excessive cell division and growth. Therefore we might reasonably interpret the question in terms of intracellular fluid pH and ignore the pH of other materials/fluids in the body.
Cellular pH is regulated. The body has mechanisms that maintain the pH of cells independently of other internal and external factors.
However Cellular pH does play a role in tumor cell survival 
Can we deliberately alter our cellular pH?
In general, no. We cannot easily or safely manipulate our cellular pH outside normal ranges. Our cells have mechanisms to regulate pH. If pH goes outside the normal ranges we become ill and may die. 
Intracellular pH is difficult to measure and may vary in different types of cells and in different parts of cells.
pH of the plasma (i.e. pH of the plasma of whole blood = conventional "blood" pH) is controlled at 7.4 (7.35 - 7.45).
From D.Brooks, University of Nebraska-Lincoln
The normal human stomach has a pH which can range from approximately 1-3 but is usually closer to 2. When there is food in the stomach the pH can raise to as high as 4-5. After the food leaves the stomach bicarbonate ions are secreted to neutralize and alkalinize the mixture. The pH of the small intestine is approximately 8.
From University of California Santa Barbera
It is only the interior spaces that are alkaline, whereas the interior of the muscle fibres is actually acid.
From University of Rochester
One of the major obstacles to the successful treatment of cancer is the complex biology of solid tumour development. Although regulation of intracellular pH has been shown to be critically important for many cellular functions, pH regulation has not been fully investigated in the field of cancer. It has, however, been shown that cellular pH is crucial for biological functions such as cell proliferation, invasion and metastasis, drug resistance and apoptosis. Hypoxic conditions are often observed during the development of solid tumours and lead to intracellular and extracellular acidosis. Cellular acidosis has been shown to be a trigger in the early phase of apoptosis and leads to activation of endonucleases inducing DNA fragmentation. To avoid intracellular acidification under such conditions, pH regulators are thought to be up-regulated in tumour cells. Four major types of pH regulator have been identified: the proton pump, the sodium-proton exchanger family (NHE), the bicarbonate transporter family (BCT) and the monocarboxylate transporter family (MCT). Here, we describe the structure and function of pH regulators expressed in tumour tissue. Understanding pH regulation in tumour cells may provide new ways of inducing tumour-specific apoptosis, thus aiding cancer chemotherapy.
From Cellular pH regulators: potentially promising molecular targets for cancer chemotherapy.
Cells must constantly maintain their pH in order to function properly. In animals, for example, the maintainence of blood pH is crucial for life. A slightly acidic pH (6.95) would result in coma and death. A slightly more basic pH (7.7) would result in convulsions and muscle spasms.
From MAINTAINING CELLULAR CONDITIONS: pH AND BUFFERS. University of Tenessee, Knoxville