jatofau1,
Ideally, you would need to run a car on your track and then run the same car, untouched, on the intended 42' track. From that you could figure the ratio then use that ratio to estimate where you need to be back on your track. Without at least that, it's a crapshoot. Too many variables, as Kinser said.
As an example, we had a scout car, using the slots, stock wheels and axles, four touching, on graphite, go 2.880 on a 42' Besttrack. Sound crazy? They had a laser beam start positioned slightly down from the gate. The first car down broke the beam to start the timer so the cars got a pretty good running start which made the times abnormally low.