Engine jumps around when trying to start it

Old engine jumps around when trying to start it, timing, air, do not really know. Found out that engine had a lot more wrong, cracked head or something else. I decided to go to another engine, I put an 2002 into 2003 body, found that I had to use center intake from 2003 engine for the plug-ins to be the same, 2002 and 2003 a little different. Anyway, set distributor and timing, new engine acts like the old. I replaced cam position sender, changed the coil, engine turns over smooth now, but still will not run. I guess just a little after you let off the key, but it will not pick up and stay running, something is the same from old to new, I just need a new direction. Thanks.