I have a 90 Chevy truck that starts when it wants and that's not always. It gets spark but what else could it be it's a brand new motor what else should I replace. It's a 350 vortec with a throttle body

I would replace the Ignition Control Module (ICM) and coil, which are not expensive on most Chevy trucks of this generation, if they have not been replaced in the last 120,000 miles or 10 years. ICMs, particularly, are problematic on these trucks; coils less so. You may have spark, but the plugs may not be firing when they are supposed to. Easy job.