New 350 problems HELP!

I have a 95 Z71 throttle bodie. I just bought a brand new crate engine. I dropped it in and attached a set of vortec heads w/ a brand new manifold and headers. The damn thing wont crank:banghead: . Every thing is brand new except for the altenator, a/c pump, And power steering pump. I have checked the firing order a dozen times. The starter drags unesseceraly slow but it acts like it should still crank. I know its getting fuel cause i used a 1/4 tank of gas trying to start it, plus the injectors are spaying fuel and fire comes from the headers. I did however forget to ground the engine and tryed to crank it, and i got closer to starting the damn thing than after i grounded it:screwy: . Now that ive grounded it it aint hittin like it was before. What did i tear up? Also when it does hit it back fires through the throttle bodie and catches fire for a brief second:confused: . I set the valves the way Haynes said to?!? One more thing now mah starters fuckin up when i turn the key it just klicks and i lose all power to every thing and i have to dis- and re- connect the battery to get any kind of power. It kinda started doin that all of a sudden cause it WAS workin fine(slow tho). Can any one help me i am so fed up with this:nono:.

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).