A personal Postscript. Sometime prior to 1946, my father, Alexander M. Malcolm Jr., built a tractor from a Model T truck using plans he'd found in Modern Mechanics, Popular Mechanics, or some similar source. He made some modifications along the way but the result can be seen in the photos below. That's Dad steering and my older brother Don going for a ride. In the second photo my grandfather, Alexander M. Malcolm, Sr., is guiding the plow. Both photos were taken in the spring of 1946 on the family's 2-acre plot in the West Hills section of Huntington, Long Island, New York. (We sold the property about 1959 and 14 houses were erected on it. Only the gate posts remain. Of the tractor, only the seat and steering wheel were salvaged.) The tractor features the standard Ford 2-speed transmission with a Chevrolet 3-speed tacked on behind. That gave him 7 speeds forward and 5 in reverse. (Try the math; it works.) The truck chassis was much stronger than the sedan chassis the plans called for.
Recently, I found the plans he must have used online, here: http://www.vintageprojects.com/tractors/model-a-tractor-plans.html Fearing that the site may change locations or disappear altogether, you can also view the plans in PDF Format here.