New header pipe suggesstions to complete my mods.

[Transmatic]

My 2007 400ex was stock. I'm in the process of installing these mods while nursing a knee injury.
Modified air box lid and snorkel
K&n air filter with outerwares
White bros e series exhaust

I just realized the stock header pipe doesn't fit together flush with the white bros exhaust. I already have the carb pulled to install a 42 pilot and 155 main jet. Rather than fooling with fitting the stock header pipe to the exhaust I'd rather go ahead and purchase an aftermarket header pipe. I'd rather not spend $250+ but if that's what it takes to get the best bang for my buck then so be it. I mainly ride trails, private dirt roads, clay pits and some MX style but not much.

Looking for suggestions on best header pipe with above mentioned mods. Should I go to 165 main jet? I ride at about 700-1000ft above sea lvl.

[400man]

an aftermarket header probably wont make much difference with your mods. if you had a bigger cam and higher compression piston, and a bigger carb like a 450r carb, then a bigger header would help you out. a common mod is to grind the welds on the inside of the stock header to make it flow better.

[Transmatic]

I hadn’t considered grinding down the welds, maybe I’ll just do that. As you touched on, I don’t have many mods, and don’t plan on doing any more until next winter.

Any suggestions on how to fit the White Brothers e Series exhaust to the stock header? There was a noticeable gap when I tried to fit them together for the first time last night. I'm thinking I should still be good with the 155 main jet?

[LxMxL97]

There should be a spacer/adapter that makes up for the gap

[Transmatic]

There should be a spacer/adapter that makes up for the gap

Wow. I def don't have that. I've had this white brothers exhaust laying in my shop for about 8 months and now I can't even install it. Nothing can ever be easy. Maybe I'll just buy a different slip on exhaust. I like the HMF pipes and I'm not stuck on the white bros.