my personal opinion is to go the 60 series upgrade:
pros:
it removes the bell crank on the front crossmember,
removes the drag link that runs along the side of the chassis rail under the shock(usually rubs the shock when you lift)
requires upgrading swival arms to 75 arms (stronger)
requires upgrading the tie rod and drag link to the thicker 75 series rods
possibly better geometry when moving the front diff forward
a good excuse to upgrade shock towers to F250 towers
could upgrade to a 60 or 75 steering column moving wiper controls to the steering column (personal preference)
You can use your existing power steering pump
cons:
requires a 55 steering column if you want to keep the 40 steering in the cab which are getting harder to find
requires drilling, crushtubes and backing plate when mounting the new box (not a simple swap)
requires notching of the inside webbing on your shock tower to allow the intermediate shaft to pass by
requires removal of shock tower to do above (pain in the ass)
if you mount the 60 box and then move your diff forward it could affect steering geomtry adversly
cost for bits (i have probably spent about $800 total on the upgrade, including a better non leaking PS pump but sold my 40 powersteer box,pump and column for $600)
depending on your lift you might need to swap the 60 pitman arm with one from a 70 (more $$ and sometimes hard to find)
but once you've done it i reckon its well worth it.. MY 40 power steer used to bind up in places and I think because of the better geomtry the 60 gear is less likely to do so.
this is just my 5c worth. pics of my upgrade process are here:
http://www.phat42.com/gallery/v/the40/60steering/ and my build thread:
http://www.outerlimits4x4.com/ftopic139548.php