Yeah, since a four stroke doesn't really have any room for a nice two stroke up-pipe, we went with the down pipe.
I don't see how this is at all correct, unless you are determined that it's either to be a definate 'UP' pipe (like what is shown on the XR chassis with the 2t AC Honda engine), or a down pipe. 2ts for about 20 years have had 'low boy' pipes - mid height pipes. And there is room aplenty for that type of pipe. Perhaps that is what you are calling a 'down pipe'?
From the picture in your article, it looks like you've used the 06 YZF250 chassis (wrong on that, I checked further and it's the older 426 chassis, so more room again).These dont' have any form of drop tank, and perhaps a trim of some barrel fins (and it's easy to add length to other fins), and a different KS lever to clear the end of the convergent cone, is all that you need. I've fitted a short , Gas Gas lever to a CR500 knuckle for my purposes, so it's an easy enough thing to do if you can't find another bikes lever to fit directly on.
The engine doesn't look high, so, if it is a low pipe you're doing, you've either got a long header (perhaps ovalized) under the engine, or you've a pan cake divergent cone, belly section and convergent cone.
Although, you could be playing semantics and doing a snail pipe, with the stinger and muffler under / exiting the engine.
I'm waiting with much interest to see the final thing - without a doubt, a great bike could resuilt - many of the AC 250s (and other sizes) were wonderful engines, that could easily match up to a modern 4t, whilst still being as simple as an axe.
