This is what I'm getting at.
The easiest way to do that is a turbo. An M90 build pushing that much will have much more money in it.
The last time I checked the current record for a NA L36 was ~270 WHP, but that didn't have an accessory belt on it. Put one back on, and you'll be around were wes blair was, 240-250 WHP IIRC. That was however for a stock bore, stock crank.
You mentioned different firing order, so I presume you're having a new one off crank ground? The existing stroker crank won't work for that. Now I'm curious, who's making the crank for you?