Can someone give me an accurate measurement between the two points in the picture? It doesn't matter if its measured on the stock manifolds or a header just as long as its from an EJ255 or EJ257. I think the header I have is too wide by 1/2".

 

are you sure you are putting in all 6 studs (both side) through the hole at the same time and not one side at a time?

 

I did try to do it one side at a time and now thinking about it that could be the reason but I would still like the measurement before I tear the stock one off again.

 

Loosen that mounting up in the middle of the headers.... mount the headers to the exhaust manifolds carefully.... then tighten it back up. Should give you a good fit. May have to hit one side of the headers with a rubber mallet to get it lined up correctly. Then tighten it down.

 

The middle was definitely loose. I tried hitting it with a mallet as well, it's as far in as it goes.

 

Here's some new pics. I went ahead and took the stock manifolds off and measured them. The new header is off nearly 3/4" and that is with the header as far into the slip joint as possible. The stock manifold measures 16.5" across and the header measures 17.25".