Gday new to this site just bought my first dato and had head gaskeet crack and now sized pistions advice or help asap??????????????????????

No need to fit over-sized pistons. If it is only a blown head gasket, prep the block face and head face well, and fit a new gasket.

i have skimeed the head all is good in that department complete new head gasket kit installed. but wont turn over im thinking that the pistons have seized and i dno how to free them ? or if im totaly stuffed?

With transmission in nuetral, you should be able to turn the engine by hand either by:
* turning the fan, which is difficult
or
* by a wrench on the crankshaft bolt
If it won't turn, remove the starter and try again. If it still won't turn, disassemble the engine and find out why.

hmmm ok ill give it a go thanks for your help !!! i have a feeeling im stuffed know anywere good to buy a twin cam ???

yeah down at your local car delaer, but it comes with a rubbish car, not a datsun

Going on a little further from dd's advice, I always found it much easier to turn the engine over by hand by removing the spark plugs. That erradicates all the resistance (compression).

You really need to have a good look before you declare the engine seized.

You'd need to give a bit more information to get a reasonable response.