If it was mine... I can almost guarnantee you that I'd go looking for a newer tractor like a TW or even an 8700... just for the simple fact that you're getting a nicer cab. That's from a practical standpoint.
If ~this~ 8000 is of some great importance... then I'd probably go looking for a used engine. That may or may not be easy to find. Places like Fawcett tractor wreck a lot of Fords and they more than likely have a 401 around in stock. HOWEVER.... one needs to make certain that it's the correct 401 with the rear drive/sump oil pump vs the newer style front pump. New engines will not fit behind the front bolsters of the 8000.