A peice of wooden dowling the same length and diameter of the stock spring will keep your forks nice and stiff. A rigid fork will be lighter and look a lot better than a crappy old suspension fork.

Tiny differences in fork length are not a big issue. THere might be a more noticable difference in handling going from a non-suspension corrected rigid fork to a 2" (50mm) or more lontger suspension fork, but even that is usually not terrible.