I think you could make the TC4 motor mount fit, but you would have to do considerable Dremmel work and drilling. I would get the TC4 chassis. I converted my whole car to TC4 for about $85. See the TC3 thread search for TC3 to TC4 conversion for part numbers.
and there is a followup post that this one links to with a picture of the mount and the converted car.
This new motor mount and its long brace is very similar to the one I constructed and showed to Barry Baker after he raced in Houston.