I'm not on the Ls bandwagon (at least not yet), but I've got a few ideas.
Moving to the Ls platform, no matter what size, gets you into a more modern design. Namely a better set of heads. Which...IMO is where the power is at. They also have strong bottom ends. And according to some (especially if you are member on YellowBullet) they can be had for nothing and make 2000hp with a cheap eBay turbo. Now in reality...that doesn't happen. Around here if you can find a running 5.3 with all the parts that doesn't have 200,000 miles and cost $1200 you're doing good.
I think the cheapest way to get an Ls running on carb is and intake and an HEI (I think DUI makes has a SBF one that drops in). You don't need COP to run an Ls motor. Quick google search brought up a Chevy High Performance article,
http://www.chevyhiperformance.com/techarticles/0508ch_chevrolet_ls1_carburetor_power_increase/