If you get the " shorty " or " block hugger " type headers should have plenty of room. Not sure about the Alternator mounting although there are many after market bracket setups available. Headers flow a lot better than the rams head cast iron pieces, although with a stock 283 wnon't make a huge difference. If you go headers pay the extra $ for the ceramic coated ones - the chrome plated will eventually rust. Ceramic also dissipates heat better.

1940 Chevy KC 1/2-Ton
In the Stovebolt Gallery