did u by any chance line up the camshaft and crank sprockets? dot to dot. another thing. another fool proof is using a wire tie down the spark plug hole. when it reaches the very top it should be TDC, try it if it still pops, its 180 out. this means take the distributor hold don clamp loose and remove. lift the distributor and turn the rotor 180* around from the initial spot it was at. give it a try.
if none of the above works. your valve adjustment may need some checking. u may have your lifters bottomed out.
this may require u to loosen all the rockersto a snug fit. try cranking it and if it cranks let it run a lil bit may get some oil everywhere though. this will pump the lifters back up where they should be. If u dont mind the oil u can adjust while running. tighten till the clatter goes away and give it another 1/4 turn.