Ah, ok, you have more or less a VPN setup going on there. It looks like all the way back at the provider, they're taking connections coming into your IP and forwarding them through an internal tunnel to your box which resides at 10.8.0.1.
So you're saying you want people to be able to connect to YOUR box on port 3074, and then be able to access a box that your box is able to access which is located at 10.8.0.6?
This might be possible, and I could give it a try when I get back to my network closet later today, but like I said I'm not sure if the router can NAt a connection that comes in and goes out of the same interface.
You COULD in theory build a simple proxy using nc (netcat) that will accept connections on your box and blindly forward all traffic to and from your destination IP, but this will probably end up being more work than it's worth and not necessarily that reliable.
If you have clients that SSH into your box, you can instruct them to use SSH forwarding as a workaround. For example, the user would execute:
ssh -L 3074:10.8.0.6:3074 email@example.com
and then they could connect to localhost:3074 to access that box.
Nearly all Windows SSH clients offer a port-forwarding option as well.
The only downside to that is of course a person must login to ssh and stay logged in when they're working with the other box.
Will get back to you once I've had a chance to test this.