Pre-'69 W series Sweptlines with the bell housing cross member are
very difficult to use headers on. I know of one type of header for
the A318 that works. Aside from that, my suggestion for your 383
would be to investigate a " shorty block hugger" style header or get a
pair of cast iron big block [Sweptline] exhaust manifolds. Headers are
kind of a pain in the butt anyhow, with their leaky flanges/collectors.
I recall a couple of guys have fabricated their own [i.e. stretched them by adding tube length]
More work than I'd care to get into...
John