Stock header length for the bike is 32"  the range for a 650 cc bike should run between 30-34"  shorter pipes favor higher rpm ,longer favor lower. You should design for the rpm range you'll be riding at most of the time . You might run a short pipe under the motor with a square muffler , I think I've seen these on Ryca Sportster kits. What you've planned looks to be very short which might be OK for WOT but produce a bag sag in midrange , and dumping in front of you might not be great for your hearing.

You aren't really looking for an engine that makes top HP......so you can try a short exhaust and see what happens.  The issue that I see is that with such a short system, there is very little room to do an pulse dampening and almost anything you try is going to be pretty loud.

I can see the photo/sketch on my computer....here it is using tinypic.

For performance, you want the exhaust to go out the header with a high velocity, and that rushing exhaust helps to scavenge the burned fuel out of the cylinder.....and it may even help to allow some movement of fresh fuel/air as the intake valve opens.  If the rushing exhaust has no place to go before it is restricted, or if you make a large diameter pipe that reduces the velocity of the exhaust flow.....the efficiency of the system is reduced.

Compared to all the other work you have done....fabricating a trial exhaust system (or 3) is pretty minor.  I think you just need to build something that looks good.....and see if the bike will run with it.
(The Board Track headers you are trying to copy were used on motorcycles that ran wide open and were unmuffled........not really acceptable for a modern street bike).

sandmanred wrote on 02/08/18 at 08:42:37:
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We cant' have that.  Fix it!  lol

The stock header does work fine....with a muffler on the end.

Open and/or drag pipes are know to create a flat spot or sag at certain rpm's in the mid-range.

(Something that didn't bother folks riding real board trackers that ran wide open all the time.....they didn't even have a clutch or a gearbox).

It might pay to know that there are two components leaving the exhaust pipe ,exhaust gases which move more slowly, and a  pressure wave that starts when the valve first opens that can travel at the speed of sound. It is this pressure wave that forms the basis for tuning the exhaust ,and the length of the header pipe. when the pressure wave reaches the end of the header it expands and moves backup the pipe as a negative pressure and if it arrives to find the exhaust valve open it can help pull out some of the remaining exhaust gases as well as pull in a fresh charge due to valve overlap . the return  speed of the wave is slowed by the fact that as the gases move down the pipe they cool .Shorter pipes favor high rpm , which is why they were used on board trackers running at WOT.  Using a stepped exhaust pipe increasing in size as it get farther from the head would allow the gases to expand and cool  slowing the wave and might help in shorting the header length so it might work better at lower rpm , but the noise would ,without a muffler ,still be painful . As a test place the stock header on with no muffler and run the bike (wear ear plugs!) Running a "lollypop" baffle may help reduce mid range sag by breaking up the returning wave ,but overall performance will suffer.