To start doing the transplants you have to actually do the transplants. Insisting on avoiding doing it on mammals, wanting to do on humans gives only one solution: would you really prefer to immediately experiment on humans instead of mammals?
Try reading about the history of medicine: after Galen (second century common era), mostly thanks to different church/ideological dogmas, "doctors" (quotes because they were quite unsurprisingly unaware of the scientific approach) spent hundreds of years theorizing instead experimenting. Experiments of head transplantations are absolutely valid experiments.
More recently and related, USSR had its own ideology-based suppression of valid scientific approach in biology
In this specific case, this work with dogs would later play a role in inspiring Dr. Robert White to attempt the same with monkeys. This larger body of work would then play a role in the development of new surgical techniques for humans, such as the use of therapeutic hypothermia that improves survival rates by dropping body temperatures (something 'played with' during head transplant experiments to keep the brains functional for a longer period of time.)