There are lots of ways to go about it. To do it on the cheap, I would suggest getting some rail extensions. You could probably go to 144 pretty easily from a 121, as I have seen 136 RMKs stretched to 156 with rail extensions.
You'll probably want a 2" track. To accomplish that you should go to 8 tooth drivers up front, to give you the clearance that you need. Yes, you'll also want a tunnel extension to go with it. You should be able to find most of this stuff in the swap, else check out TracksUSa or SLP...sounds like these up top have some gear as well...jmho