383 Exhaust Manifiolds vs. Headers

Don't roast me, as i spent real time searching this out with minimal results.

I'll just be honest, I'm disenchanted about the next to no options for headers on BRB block in my early A-body. There's a few companies that want a fortune for headers that dont even fit my 4 -speed; I then have to send them off to be modified again. Easily over a grand!

Would it be absolutely blasphemous to buy OEM Magnum exhaust manifolds? This is just a very light street rod. How much of a performance hit would i see by using manifolds? Thanks in advance.