timing 91 chevy truck

i have a 91 chevy truck i had the motor rebuilt the cylinder bored 40 over. the heads rebuild to new cam, the cam was bigger then stock all
censors and computer and tbi i used the truck runs very good rolling down the road but idles like crap cant get the timing or something right is there something special to do when settinng the timing with computer still in the truck. i have a lot of money in this truck and cant get it right need help

Uh.... I have a 94' GMC truck with a built 350 in it. 30 over, bigger cam and head work. When we put it in just 1 day ago it ran terrible and we checked the timing it was on 4 degrees. I ended up advancing it to about 8 and it runs better but still not right. It says on the fan shroud to set it at 0 what should i do?