With all the extra power I think stock is out of the question but I will be running a stock for a while to do all the original testing.
I ran stock length with the 2003 KTM 250 motor for a while.
Oh yes, it will wheelie on command at any speed and makes hillclimbs difficult.
I only have a 2" extension in it now, like it, much better control on hillclimbs and still wheelies well at speed.
My son has a 4" extension under one of his big hp Blasters. Getting a bit long, especially on slippery surfaces.
Banshee Dave has good advice.
Steve