I had the same consideration on 305 or 350. I am so very glad I went for a 350 after comparing my 280z with a friend's late 80s 305 Camaro. Granted the f body is heavier, but still, the 305 is not a sound investment for the effort to swap from the L28 IMHO.
To give you an idea, I could easily beat my friends camaro at a light to light game with a stock 280z running a tired motor.
The reason the 305 is not liked is because the narrow bore shrouds the valves, making it more difficult for the engine to aspirate.
From Wiki:
Year hp (kW) lb•ft (N•m) 1976 140 250 w/2bbl. 1977 145 245 w/2bbl. 1978 140 240 w/2bbl. 1978 160 235 w/4bbl. 1979 130 245 w/2bbl. 1979-Ca 125 235 w/2bbl. 1980 155 240 w/4bbl. 1981 150 240 w/4bbl
Also, if I am not mistaken, the stock heads on a 305 can not use TPI, but instead uses an intake for chevy's 'crossfire' Essentially two huge injectors at the air filter.
Just my vote of not worth it.