I think you posted in your other post that the original wheels were zero offset right?
So wouldn't you ideally want a 42mm spacer?
Now lets see, 42mm = 1.65354331 inches
At zero offset, you'd have a backspacing of 3.75 inches. I believe if you search you'd be able to find the backspacings that people run. I know that the number of smallest backspacing people have ran is in previous posts, but I think a generic 2in spacer would work, in fact a 1.5in one would probably do the job as well...
Mario