Go from the bottom, yank the trans crossmember to make your life 100 times easier, it's only a few bolts, wiggle the headers in and bolt up. You will have to hold the back of the trans up with a jack or something to get the headers in place.

 

Driver side from underneath, passenger side from up top.

 

from my experience installing pacesetters and o/r y on my sierra, you dont have to pull the crossmember down, and if you jack it up high enough the right side will slide in from the bottom without having to remove any heat shields, the only thing i unhooked on the left side was the steering arm, also i put the y pipe in place before installing the headers so once the headers were up everything slid into place. the 3 bolts that hold the factory manifolds are a real bitch so i just cut the y pipe up in order to drop the manifolds out

another word of advice is spray the **** outta the manifold bolts, i sprayed mine twice a day for a week straight before the install with pb blaster.. also another trick a welder at work taught me was to tap the wrench with a small hammer when breaking the bolts loose, now i didn't exactly get away with 'tapping' the bolts but i didn't break any bolts off in the head that way either and this was a 200k+ miles truck

one more thing: get band clamps! u-bolt clamps suck and make taking it back apart a real pita.

 

^^^ this is what i did as well, if you are going to scrap the old parts just use a saw to cut up the exhaust for easier removal.

 

I just installed OBX Stainless headers and y-pipe with no issues at all....didnt remove support either...had a floor jack and jack stands and put my 08 Sierra as high off pavement as possible... Fits perfectly ... Saws all or grinder ( i used small electric grinder w\cut off blade) helps with demolishin of stock pipe... Very happy with mine .. Pretty sure different brands long tubes are all the same unless other brands have design flaws...Good luck... Oh and i used Liquid Wrench on my exhaust manifold bolts the night before and they came out like they were never tightened...Lucky Me...
I went from bottom with them and had no issues either

 

I appreciate all the advice and hints. Thanks much.

 

I removed nothing except old manifolds and exhaust. Installed and test drove. Very easy.

 

Lifted? If your front diff is dropped anymore than 2 inches it will hit you front drive shaft...

 

Well, I managed to install my Pacesetter headers and ORY myself on jack stands under my carport. Thanks for the hints. I decided to document my headers installation, maybe it'll help someone else. Here is the link for the install guide if you are interested.

https://www.performancetrucks.net/fo...8/#post4909235

 

trying to install long tube headers right now but getting stuck on frame and trans lines on the hump on the headers do I need to lift motor I spoke to a exhaust shop near me and he said a hammer and possibly lift the motor