hey thanks everyone for your help.

What i ended up doing was towing it down the road, left it in 4th gear and slowly let the clutch out. Wheels didnt spin at first but after about 20m the wheels started spinning in gear.
I guess it free'd up the pistons in the bore. As soon as the wheels started spinning i unhooked the car cranked it over and it started first go.

So happy !!!! :) Car is running great. Just running it in now

the wheels locking caould also be lack of weight to make them turn in gear in a ute.
you've checked spark, do you have fuel?
either take the fuel hose off at the carby and point it into a dish, then crank the motor and make sure plenty comes out.
alternatively you could spray some aerostart down there and if it kicks straight away that would also indicate fuel. Since your running flat-tops, do you know your new comp-ratio? it may be higher now and require a stronger spark, if the engines turning slowly, try jump-starting off another car to get the voltage up.

nothing to do with engine if the wheels locked up either the box or the diff is no good

Hi guys have some problems with my car.

before i start ill give you a little run down on what i did to the car. i am not too technical so ill explain to the best of my ability. (dont hold it against me :P)

I bored the block out to fit in bigger pistons, they are out of a mazda, they are flat top also.

When putting back together we made sure that we could turn the bottom pulley so the pistons were going up and down in the bore. all seemed ok.

put the car together and tried to start, would turn over but doesnt seem to have enough power. We checked the distributor to make sure it had spark and was firing at the right time.

I got a tow from a mate this morning to try and start, had the car in second gear and all that happened was the back wheels locked up. Didnt even kick over.

Took it back home and spun the bottom pulley with a shifter and it seems to be turning ok (its pretty tight) but it is turning(so it doesnt seem to be hitting the head).

BTW this is in a Datsun 1200 ute.

Any suggestions would be very much appreciated?

Thanks in advance
Milan