What performance modifications are popular on chevy 305?

Honestly, junking the 305 and installing a 350. The 305 is basically an underbored 350. Because of it's small bore, large valve heads can NOT be used because larger diameter valves will actually hit the engine block. The 305 is a good fuel economy V-8, but it simply is NOT worth messing with if you want to go fast. 305's also have weak blocks, connecting rods and crankshafts as well. Ask any engine builder and they'll tell you the same thing. The 305 is good for what it was designed for, but for performance, it's basically junk.