Building an offset, reverse rotation v-drive for high RPM's

I am trying to design and build a gear box with an input and output on the same side of the box. That will also reverse the rotation of the two. It needs to be functional at up to 6000 RPM's and work sustained at between 1500 and 4500 RPM's. The way that I see it is that the input would be a short shaft into a Helical Gear which would then engage another Helical Gear and that would reverse the rotation. Then on the same shaft as the second Helical Gear there would be a triple or quadrulple chain drive that allow the power to be transmitted over to another chain gear set that would attach to the output shaft. The input and output shafts need to be about 12 inches apart to clear the transmission that I want to attach to the output side of this gears box. I do not want any sort of gear reduction and I hope to get as many of these parts as possable off the shelf. I realize special machining and fabricating of the shafts and case are going to be required but I hope that I can use quite a few off the shelf parts.

Thank you to anyone who can help with this problem!

Lee