[silicon212]

Definition of Cranking: Starter engages, crankshaft rotates. So, your engine is cranking fine (albeit slowly). It's just not firing.

Now, with that out of the way, my first guess would be a firing order and/or timing gaffe - make sure the plug wires are set on the distributor in the 1-8-4-3-6-5-7-2 order with cylinder 8 clockwise from cylinder 1. Also, make sure that the harmonic balancer mark and timing fork on the timing cover are correct and that there has been no outer ring slip if it's a reused harmonic balancer (vibration damper). Make sure that the marks are correct, and with the marks lined up at 0 degrees TDC, check that the rotor is pointed to either cylinder 1 or cylinder 6 depending upon camshaft position - if the rocker arms on 1 are down on their valves (you are unable to turn the pushrods by hand) set the rotor to #6 (this is 6 compression, 1 exhaust stroke), if the #6 rockers are down on their valves, set the rotor to #1 (this is 1 compression 6 exhaust).