Some things I learned while trying to install a set of replacement headers for my 82 4x4 Blazer.
Step 1: Cut Off Several 3/16 X 2.5 Inch Bolts
Make sure that whatever bolts you use, that you get them with a non threaded shank. It will make it easier to thread into the block w/o cutting your fingers.. In my experience, vise-grips work better on these as well.
Step 2: Debur the Weld Slag From the Bolt Holes
The particular headers I had were moderate quality. I don't know if high-end headers have this problem. When I tried to install these headers, the bolts would not line up. By deburring the weld slag, everything came into alignment. Be careful!! The high speed die grinder can "grab" the hole and spin itself around. Make sure to wear eye protection as well. The little bits-o-metal can cause a rough night.
Grind the weld down just enough to allow the full hole to be exposed.
Step 3: Remove the Old Headers
Depending upon if you have headers already installed, or not this step may suck or may not be too bad.
It helps to squirt some PB-Blaster a few hours before you start. Breaking a bolt off in the head is a bad thing, which will result in a lot of cussing and several hours of crying..
My particular truck is missing just about all the smog stuff, and air-conditioning, making this step simple. Don't forget that sometimes things are easier to get to from the bottom. Don't be afraid to crawl under the vehicle.
Take the plugs out first too. The porcelian will crack pretty easy when you go to get the headers out.
Step 4: Clean Up the Block
I use a high speed rol-loc to clean the ports off. Make sure you put the plugs back into the cylinders so that crud doesn't fall in. You can scrape this with a scraper, or razor.
Step 5: Install Studs
Look at the headers for the holes that are 'blocked' by the headers themselves. These bolt holes should not get studs installed as they are designed to have the bolts installed as the header snugs up to the block.
After threading the studs in (finger tight), align the gasket and slip the header in place. In this case, I was able to get the old header out through the top, but the new ones had to come in from bottom..
Step 6: Replace Studs for Bolts
Working slowly, so you don't cross thread install the final bolts. Make sure to start 'blocked' bolts first.
I used hex headed (standard) header bolts. The old headers had allen headed bolts. It's my choice to use the hex head. WIth this install the headers got in the way of the bolt heads, making an allen wrench difficult.
(I lost one bolt, so I had to reuse one of the old allen headed ones..)