i have an 88 f150 302,5.0 and just rebuilt the engine and everything under the hood but now that its back together and back in the truck it backfires pullies spin and truck tries to start but wont. i have reset timing and checked all wires everything is right what do i do now please help

 

Still sounds like a timing issue if your are getting fuel. Have you checked your fuel pressure?

 

If you put new plugs in they might have given you the wrong ones. cross reference to another brand. Happened to me long ago. Once upon a time I got a 57 chev that someone had put a new cam in. But it was the wrong one. It was off just enough to make everything appear right but it would not start. Plugs a good possibility.

 

check that the distributor is not out 180*. you need to find tdc on #1, and see where the rotor is pointing