I am not familiar with the TTI headers. I've used Hookers in the past... I mean, that I have used Hooker headers in the past (don't get the wrong idea here). They have like a 1/4" to 3/8" thick flange and the threads in the head for the exhaust are 5/16 x 18. So for most of the bolts, you can use 5/16 x 18 x 3/4" to 1" length. They make special header bolts that are available at most auto parts store that have smaller heads than standard bolts to make them easier to install/remove. Sometimes they include a set of header bolts in the box when you buy new headers. Make sure to get the 5/16" x 18 bolts, not 3/8" x 16 bolts.
Now keep in mind that the end bolts on each head go into the water jacket and also the factory used studs on the end to hold up the manifold/header while the other bolts were started. I recommend using a stud in the end holes of the exhaust like the factory did. this allows you to hang the exhaust in place while you start the other bolts - much easier than trying to hold your headers/manifolds and start the first two bolts at the same time. So you want to use a stud with a 5/16" x 18 thread on the "bottom" and then have enough length to secure the header and nut on the other end. The factory studs had coarse thread on the "bottom" end that goes into the head, and fine thread in the threads for the header. You will want about 1" to 1 1/2" of length on the "fine" thread portion of the stud to be able to go through the header flange and hold a nut and washer. If the stud is too long, it may interfere with the header pipes, depending on how they are routed. Be sure to get a few different length studs and test fit with a header to see which one works best. They also make studs with coarse threads on both ends, which will work also. Use which ever is easier for you to find, keeping in mind to make sure the nut thread matches the end coming out of the head (either fine or coarse thread).
Now for the base of the stud that goes into the end bolts on the head that also are connected to the water jacket. You need to seal these threads. There are a few ways to do this. You can use a thread sealer/Loctite, pipe dope/sealer (available at any hardware store in the plumbing section), or some people use permatex silicone sealer.
Another method that I use is put them in dry like any other thread and pick up a tube of the "worlds best stop leak" at the parts store. This is a little tube with silver powder that you pour into the radiator and run the engine for a while. Don't use the Bar's leak with the pellets that look like rabbit crap - it can clog your heater core. You don't need to use the whole tube to seal the threads. I usually put 1/4 of the powder from the tube into the radiator and run the engine for over 30 minutes. The sealer will find the leaking areas and plug them by itself. You can do this while you break in the cam. I showed this to one of my friends son's had a friend with a Monte Carlo SS that had a 1/4" chunk of water pump gasket missing and was leaking. We poured 1/2 tube of the stop leak in and ran it for 40 minutes until the leak stopped. I only recommended this as a temporary fix until he could replace the gasket. He sold the car two weeks later without changing the gasket and it was still sealed! Good luck installing the headers, they can get really tight....
