One question, are you trying to stiffen the action of the forks or reduce the flexing?  The 250 front forks will not work because they are 35mm in diameter, the stock forks are 31mm's.  To use the XL175 front forks should be a direct swap as the fork tubes are the same diameter.  To use the 250 front end is a little more complicated.  You will need to use 35mm triple clamps, but to do this, you will have to use a smaller diameter and lengthened steering stem.  The easiest way to use an XL250 front end is to use triple clamps from a 1976-78 125 Elsinore.  The problem is that in AHRMA racing, this will put you in the evolution class.  It doesn't make any sense, but it will.  The steering stem will need to be lengthened about 11/16 to 13/16 in.  Then the steering stop will need to be ground to allow the front wheel to turn enough.  I have done this conversion on an SL125, and it looks and works great.  You will have to put longer shocks in the rear to match the longer front forks.  I am using this bike to ride in Non-AHRMA events.  I hope this helps.

DSK