Just adding headers without also upgrading the carb, intake, cam, and doing things like increasing compression will likely make little if any difference. To anyone that says their low compression 4.2 made a lot more power after doing nothing but adding headers, I say show me the before and after dyno sheets proving this. Peoples seat of the pants "butt-dynos" are notoriously inaccurate.
A buddy of mine switched to headers from cast manifolds without changing any other parts on a 68 nova with a GM 290hp crate 350. The car made 11 more hp on the next dyno pull. Anecdotal I know, but this was on a mild performance engine and it made very little difference. I believe the difference on a 4.2 would be unmeasurable.
Even if adding headers to a 4.2 added 20 more magical horsepower, it still wouldn't be worth it to deal with the leaks and the heat. Just my opinion.