Ok here's the story.
My truck is a 1984 F150 with a 351W longblock. It has a brand new block, internls, and heads, with a total of 2,700 miles on it, I put on every one of them. I was busy at the time so I had a third rate mechanic put in the motor, I ran perfectly for 2,700 miles. I broke it in right, changed the oil after the first 200 miles, then every 1,000 miles.
I started the truck after work hearding home. Let it warm up for 5 minutes, giving it a little gas every now and then. I was driving home and got 2 miles down the road, then it backfired alot and died, wouldn't restart. Towed it home and went thing by thing, trying to find the problem. Found out the Camshaft gear was totally ground down flat, but the distributor gear was fine. So that means the timing was messed up. So I took off the intake manifold, rods, lifters, valve cover, radiator, fan, water pump, timing cover, harmonic balancer, and timing chain/ gears. Pulled out the camshaft and tossed it. Bought a new CompCams Competition Cam, and CompCams Double Chain Timing set. Installed them and timed it right, with the lifters and all that stuff. Put it all back together and tried to start it, but theres a grinding sound on the flexplate, keeping the engine from firing. You can hear the motor firing but not enough to crank it over. Can the tranny or the flexplate be messed up.

 

I JUST had the same problem with my truck.
You described my symptoms perfect.
Ok, my problem was the distro' gears.
But my flexplate was ground down to almost nothing and made it REALLY hard to start even after i fixed the distributor.
So i believe i may be the distro plate.
Or check your oil and stuff, make sure it up to the correct level and not too full.
Also is your fuel injector/carb clean? Does it smell like it's flooding when you try to start it?
Check your flywheel too.
Are your plugs and wires still good?