One of the best bang for the buck improvrments for a 305 is a ZZ4 PROM chip. A guy named Bitflipper sells them on Montecarloss.com. Very easy to install too. As for headers, Hedman currently makes 50 state legal full length headers for G bodies with SBC engines. They are not as easy to install as the old TES and 2050s, but they will work. Replacing the stock timing chain with a true double roller chain helps a lot too.
As for LS swaps, the only truly legal LS swaps into older cars that were never designed for them are the expensive EROD crate LS engines from GM. There are some legal issues with swapping in used standard production LS engines into older cars. Most of the aftermarket just completely ignores said issues and blindly push swap products. ERODs are special production engines with special emission packages and paperwork that makes them legal for old cars.
As for LS swaps, the only truly legal LS swaps into older cars that were never designed for them are the expensive EROD crate LS engines from GM. There are some legal issues with swapping in used standard production LS engines into older cars. Most of the aftermarket just completely ignores said issues and blindly push swap products. ERODs are special production engines with special emission packages and paperwork that makes them legal for old cars.