Just measured mine. My kit was bought at least 30 years ago, so pre Bridgenorth days.
The plinth for the motor is 3" high
Width 4 15/16"
Lrength 6 15/16" with a 1" wide flange along each side, for the M6 setscrews that pass through into the tappings in the bars that reinforce the base
On mine, the capacitor was a shade too long to fit into the box, so the rear face had to be bored to let the plain end of the capacitor protrude.
HTH
Howard.