Depending upon the tub that is used, it is possible that some modification will need to be made to the frame to allow for oil pan clearance, etc. Caution needs to be used when cutting cast iron tubs since broken tubs have been occasionally reported to occur after modification. Cast iron is a brittle material and is especially sensitive to stress concentrations. To eliminate crack initiation points, generous radii must be used in corners. It is recommended to use a hole saw or large diameter drill bit to form a the corner radius where two cuts intersect.
To help minimize the amount of grinding required when removing the existing Waukesha engine mounts, I have found the inexpensive hand held band saws from Harbor Freight Tools to make quick work of cutting the cast iron.