very odd, had the engine been sitting around a while?
i'm guessing its in the car now?
its weird how it has become stiff after you fitted it,
it could be the g/box? have you tried taking the gearbox off and turn the engine over by hand with no plugs fitted?
have you used standard clutch and pressure plate?
Its quite hard to work out what it could be without seeing it.
Might be worth taking the engine out again and check it over, check the g/box as well (allot of effort though especially if your short for time, but the days are longer now
).Without the plugs in the engine should turn over relatively easy with an 18mm crank spanner on the crank pulley bolt.
i'm guessing the timing is okay, if the timing is off you could be trying to turn the engine over against an open valve. generally not a good thing to do, however unlikely but the imagination wanders when you cant see what your trying to diagnose
