Very thoroughly researched story, Steve
But you've fallen into one trap...
Pat Hoare didn't stop racing the single-seater because of any change of formula. The last season he raced it was 1962, and the new limit didn't come in till 1964.
After 1962, he knew he'd need a rear-engined car to remain competitive, but when Ferrari could not, in the end, supply one, he decided to retire from racing.
He tried to sell the front-engined single-seater but there were no takers. It was then that he decided to convert it to a road car
Also, Don Macdonald didn't own the car - the record attempt was while it was still in Logan Fow's ownership