I removed a set of long tube headers 2 years ago. They were warped and leaking, no hope for them. They were probably cheapies, they were on my 65 when I bought it. My heads were stock with a small cam. I could tell that the top end power dropped about 10% or so when I switched to the HIPOs.

They are fairly easy to install. You may have to grind off 1/4" of the top of the drivers side motor mount. I had to do this, others have had to do this as well. It depends on which mounts are on your car.

If you are looking for more power by adding these, then I would suggest that you pull the heads as other have advised. Port match the heads to the exhaust manifolds. I did not do this and should have. However, I am not sure how much you will gain with a stock cam, but it still should be noticeable.

These will not hook right up to your existing exhaust.

I don't see how long tube or tri y headers will be helpful beyond the HIPO manifolds without a bigger camshaft, and this is another subject. As stated earlier, my 65 had a bigger camshaft in it so the long tube headers were more helpful.

One final note, the HIPOs look a lot cooler than the factory manifolds!

 

I removed a set of long tube headers 2 years ago. They were warped and leaking, no hope for them. They were probably cheapies, they were on my 65 when I bought it. My heads were stock with a small cam. I could tell that the top end power dropped about 10% or so when I switched to the HIPOs.
'Though I couldn't put a number to it, sounds like my experience. I decided I spent so little time at top rpm it didn't matter.

They are fairly easy to install. You may have to grind off 1/4" of the top of the drivers side motor mount. I had to do this, others have had to do this as well. It depends on which mounts are on your car.
I've had to do that on other cars. My car has the earlier 289HP mounts, which clear.

If you are looking for more power by adding these, then I would suggest that you pull the heads as other have advised. Port match the heads to the exhaust manifolds. I did not do this and should have. However, I am not sure how much you will gain with a stock cam, but it still should be noticeable.
My heads were 65's, with no smog bumps. They weren't even badly flashed at the gasket surface, which I have seen. I could still tell the difference before I got to the street.

These will not hook right up to your existing exhaust.
Yep. They exit lower, as part of the freer flowing design.

I don't see how long tube or tri y headers will be helpful beyond the HIPO manifolds without a bigger camshaft, and this is another subject. As stated earlier, my 65 had a bigger camshaft in it so the long tube headers were more helpful.
I have the C3OZ-6250-C cam. I have seen similar results with the C9OZ-6250-C, and aftermarket cams.


One final note, the HIPOs look a lot cooler than the factory manifolds!
This car was restored from rusty junk, and the engine, stone-stock except for a C9OZ-6250-C cam and HP manifolds. Shelby ran the PS hoses downward, as though they intended to do that. Totally transformed the cars' performance:

 

Absolutely beautiful mustang!!

 

Not mine, but I did see it in person. Here's what it looked like the first time I saw it:

 

will Hipo manifolds and h pipe work with the regular V8 Z bar?

 

Only if you cut the lower arm off, forge it to the correct shape, and weld it to the extreme engine end of the pivot tube. I'm not being sarcastic, I speak from experience.