I just finished installing an APN header in my 99. It wasn't without obstacles and had to deal with interference between the intake & header. First time I fired it up it went WOT and I shut id down immediately knowing there's a massive vac. leak. Tore it all down and discovered that the lower bolts were only finger tight even though I had torqued them down. It got me thinking that possibly starting it vibrated the intake into the proper position when I started it. This time when I started to torque it down I heard a distinctly audible "pop". I had done some grinding on the #1 & 6 intake runners and massage #6 tube with a large hammer so there wasn't any tube/intake interference. I'm guessing that there was a bit of header flange to intake interference that prevented it from seating properly. Just thought I would toss this out there for someone in a similar situation...

 

Thanks for sharing. Always nice when someone posts a follow-up.

 

On my 99 I had the older intake and did not need to do any of the modifications I expected to to have to do to make things fit.

I did get a loud pop on my first install. It was caused from the intake not sitting correctly on the alignment pins in the head. I broke off one of the tabs. This also caused the WOT in initial start.

A little JB weld put the tab back on long enough to get everything all lined up and bolted down. I triple checked the alignment pins on the second install.

 

So what header DOES fit with the 99+ intake without modification??

 

OEM manifold.

 

I was able to fit a Gibson header with a 99 intake without any modification to either one.