Just finished replacing the head gaskets, several codes came up at first, but after a few short drives, these two keep coming back.

When I first crank it up, the rpm's fluctuate between about 700 to almost stalling and finally settle at about 400 rpms.

IAC valve?

This motor should idle at about 650, correct?

 

The 1514 is a Neutral drive gear fault, the 1551 is you IAC.
I would suggest you check the plug connection to your idle control, lean the t/body and the idle control and see if it settles down.
The 1514, I have never seen come on before and don't see how it would effect your engines idle.

 

I am having this code as well and same symptoms, how did you fix it?