From your description of the motor straining and hand cranking being hard in part I still believe you will find this to be in the motor, a cracked gear, gummed up or bad brushes. I think the next step is remove the motor and inspect those aspects, run it off the car.
Julian
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.