I 3rd the starter fluid down the intake idea, just remember to use only a little bit of that stuff, it can be dangerous to you and the motor. If it starts or sputterers for a few seconds on the starter fluid, that tells you that spark and compression is there or close enough to run. I would look at the fact that you said the injector noid light is not flashing brightly while cranking. It sounds to me like the possibilty of a bad ground exists. With the injector wire unplugged, measure the voltage across the hot side of the plug and a good ground while just leaving the key on ( it should be 12 volts). If it is, then you have a good power connection going to the injector. Remember, the e.c.m. pulses the ground side of the injector, the hot side is always hot. Checking for a good ground signal to the injector is not that easy. Being that it is pulsed by the e.c.m. , the easiest way is a noid light ( which you said is barely lighting when cranked) . If the light is in fact dim, and you have 12 volts present, the only other possibilty is that the ground is not good. You can check this by using a d.v.o.m. with a memory feature. You can hook up the meter across the 2 terminals of the injector plug, start the memory feature on the meter, and crank the engine over. You should have 12 volts present, if not I would check all the grounds or possibly you have a faulty e.c.m.

Ursus,
I would pull a couple spark plugs out to check fuel on them(Wet)to eliminate lack of fuel and or spark.I think the distributer should be set at 0 degrees due spark controled by crank sensor and ecm(I think).If your clyinder are to dry (no oil) you will not build enough compresson to fire a clyinder like blue thunder said.I node light on a injector should be fairly bright on a injector connector on start up.O yeh Fresh paint makes a good insuslater for a ground .Clean off where you grounds wires bolt up
Dave

FILL THE FUEL FILTER!
 

I second the thought on the fuel filter. On my last rebuild (502 carb) I cranked and cranked and got nothing. Filled the new fuel filter and she fired right up and ran well. Timed it by ear and she purred like a kitten.

That fuel pump went out on me that year so it was probably week.

Ron

Thanks for all the replies

The Problem:

The manifold/injectors had been sitting disassembled and dry ( with no gas) awaiting to be put on this new 502 long block for a while ( over a year). The injectors were stuck closed. Sent them out to be un stuck, clean,flow tested, now all is well.

FWIW to test for this we cranked the engine with a fuel preasure guage on the log till we got 45 psi. Then disconnected the fuel pump connector and cranked some more. On a normal engine the preasure will drop as the injectors open, stuck injectors the preasure stays up. Learned this talking to a merc tech rep, they were quite helpful.