Headers vs. Manifolds

I'm gathering parts for my 383 swap into my '75 Duster and have a question regarding exhaust. I have an opportunity to buy some '70 B-Body HP exhaust manifolds and am wondering how much performance loss and I can expect over a set of headers. TTI headers are not in my budget and I don't like the idea of creating "clearance" on a brand new set of headers. I'm going to pull apart the 383 that I bought, but some of the specifics I do know are that it has ~10-1 compression, ported HP heads (452 or 906), TM6 Edelbrock single plane, 750 DP DaVinci carb, and a fairly radical (I've heard it run) hydraulic camshaft. I will be running a 727 w/3000 stall and 3.55-4.10 gears. I know this combo has headers written all over it, but the appeal of HP manifolds has me leaning that direction. Should I expect a 20-30 horsepower drop using manifolds and a necked down 3" exhaust with flowmaster mufflers and turndowns?