New Guy from Nebraska , HELP !!

[JimEnic]

Hey all, I have a 16 year old grandson that a friend of the family gave him a 1976 GMC C2500. Originally had a 425 motor but has a 350 now.
Anyway, grandson wants me to help him rebuild this thing. It has rust in usual places but I have done some rebuilding in my younger days. ha ha
So that isn't my issue.
I have some mechanical abilities as my son and I use to race Hobby Stocks on a local dirt track.
My problem is electrical. When you try to start it the lights on dash come on but turn it to start and nothing happens. I replaced the switch where the key goes into, but still won't turn over. We did a compression test on motor using a remote starter button so I know the starter will turn over the motor. Plus, he drove it to my house so I know it runs.

Any Help would be greatly appreciated
Thanks