im trying yo fit the engine in and have space fore the steering exhasts and etc.... how level does the engine have to be on the horizontal plane front to back

my engine block # is 3970010 this # is on the engine on the drivers side above the bell house what is it and what year.

i am also confused about another aspect the lower pully mounts onto the harmonic balancer and that mounts onto the crank on my engine it looks like there is a bolt missing right there in the centre. the shop i got this from says no!! man i looked in there and there is thread and all please help....