Idle intermittently high

[K10-Kansas]

Edit: Here's my engine https://www.summitracing.com/parts/m...xoCKRsQAvD_BwE

Hi. I bought a 1971 C10 GMC and had a 350 crate engine put in. When you first start it it idles at a high rpm and if you try to put the truck in reverse or drive while it's doing this, it dies. Letting it sit and idle at a high rpm for 1 or 2 minutes sometimes let's it move to a normal rpm and sometimes not and it will just continue to idle high. Tapping the gas also sometimes moves it to a normal ramp but sometimes not.The out of state mechanic says that this is normal and is trying to get out of doing warranty work. Honestly I give up on him anyway and don't want to drive a state away for fixes. I'd rather fix it myself. What could be possibly causing this high idle?