Place the insert in the manifold, and tap it in with a rubber mallet. It will get wedged in, so you'll need to gently pry it back out, but after you get it back out there will be rub marks left where you need to grind the insert down. I personally used a dremel with some type of abrasive disks, but even sand paper or something similar would work fine. The key is to go slow and only take off a little at a time. The L26 manifold seems to contact the L36 insert at the top. It seems that the manifold curves in sooner than the older design.
Now for all I know I did something wrong and it can work. Tim said managed to get his to work (congrats by the way!), so that's entirely possible. If you can get it to fit, you really want to put the gasket in place and see how well the insert lines up with the openings. If it doesn't line up closely, you'll have turbulence due to the air hitting those flat spots. You'll probably still get more top end, but you won't retain as much low end power compared to it fitting flush with the gasket.
