I haven't decided if I should stay with stock exhaust manifolds or go to headers. If I decide on headers, I need advice on the easiest to install, Ones with no interferance to original inner fenders or parts. any help is appreciated.
Headers will give you the best performance, and the most pain. I have a set of Doug Thorley's on my 69. They fit pretty well and have nice thick flanges that are good as far as sealing to the heads without leaks, but I still had to do things like relocate the brake distribution block and it looks like I'll still need to dimple one of the tubes a bit on the passenger side to get a little room for one of the lower control arm bolt heads. It just barely touches now, but with the engine moving in the frame when the motor mounts flex I'm concerned it might rub a hole in the header tube.
There are other options. I hear from pretty good sources that "Mad Dog"
headers both fit and seal. I didn't learn of them until after I'd already bought mine and invested in having them coated. There's also a guy on another forum (either Pontiac Zone or Performance Years) who's in the process of making Tri-Y headers. Because they're 4 into 2 into 1, they should fit much better and not have the same clearance problems that full length 4-tube headers have. You'll find opinions that say they perform better/the same/not as good as 4 tubes, and I don't know which of them are correct. I suspect like everything else, it depends a great deal on the other characteristics of the motor (displacement, cam timing, exhaust flow velocity, exhaust port design, rpm range, etc.) as to how well they'll really work. You can save yourself some grief by not trying to go huge on the tube size. There most definitely is such a thing as having primary tubes that are "too big" for the motor. Tubes that are too big make the exhaust gas flow slower (bigger tube - gas doesn't have to move as fast in order to pass a given volume), and that does bad things to low end torque because now the engine has to expend energy to push the gas out instead of the higher flow velocity helping to 'suck' it out. More engines have lost power due to tubes that are too big than due to tubes that were too small. Unless you're making north of 600HP on a Pontiac, you don't "need" 2-inch headers - they'll actually hurt you.
I chose headers because I've never had them before and I thought I wanted max power, but I tell you, if I had it to do all over again I'd probably think pretty hard about it before I chose them over the repro ram-air manifolds. I might still pick headers, but I'd sure think about it more.