141" is going to be somewhat limited, most are 136" or 144". Hartman, Mountain Performance, Tracks USA are a few to look at. Talk to Richierich on here, he sells almost anything you would need.
If you want a stock 141" tunnel extension I know someone who has one, pm me for the info if you decide to go that way.
It's 3" inches total which equates to like one more lug on the ground. Can't see where you'd ever notice the difference and the 144's are a lot easier to find.
yeah for some reason I thought you were going from 136 to 144, just re-read it and see youre going from 121 to 144. sorry for confusing you with the 136 part
dont worry about it at all i like ur idea for that and ive been looking i think now that i know where to look i might do this to my next sled instead and keep this one short track for radar runs