alright upgrading just the front brakes to LS1 brakes. Now do i have to bleed my system or is it ok to just bolt them abck together?

And if i do have to bleed the system would i have to bleed the rears also or can i just do the fronts?

 

Since you are removing and replacing your front calipers, you'll have to disconnect the brake lines, thus introducing air into the system. Therefore you need to bleed at least the front system. I suggest you bleed the entire system to get fresh fluid in there.