I have attached a photo of the third mount from the front on my Frankentruck. ('40 cab on '47 frame)
I ended up assuming the the best way to install this irregular block was with the cut-off corner inward and to the rear.
The attached photo is of the passenger side, the front of the truck is to the left. (The fuel tank strap bolt is at an angle because there is no fuel tank at the moment!)
Nick 1940 KC Model 1/2 Ton Pickup Project (with '37-'39 Bed and what we think is a '42-'46 Frame) Taos, New Mexico
