Okay this kinda changes things for me a little. Id like to make somewhere between 375-400hp but retain decent mpg and ability for long highway drives if possible. I am working on a pretty tight budget which is why i figured a ls1 swap would be cheaper than rebuilding the 6.0. the original plan was to drop the ls1 in it (which i found for $1k) and rebuild the 6.0 over time.

How much will it cost to have a shop go over the block?
should i have it bored out?
Can i put ls2 heads on a 6.0 block? or is there a better gm stock option for heads than whats on the truck?

BTW i know nothing about the 6.0 motors