First I made a plate out of some ABS plastic. I used a clear piece of plastic as a template so I could see the old EVX-2 mounting holes. Then traced that on to the ABS plastic. The plate mounts to the chassis using the EVX-2s mounting holes.

Then I placed the VXL-3s ESCs side by side. They had to be offset to fit.They actually screw down.

I made my unslow (parallel) harness a bit different. Notice the ESC wiring for reversing the rotation of the front motor. I used a servo Y harness for the ESCs.

Right now I'm running 18T pinions. I have not had much time to adjust gearing. Program the transmitter to the ESCs first thing.
Why do this? A twin VXL set up? Its waterproof! Also if you have the motors or part out a couple of Slash 4x4s the motors become very cheap. This almost cost me almost nothing. Ebay fees if that. If you have enought to buy a Slash 4x4 then you can part it buy another and then youre done. Its cheaper than buying 2 VXL combos for $260.