let's say you have a guitar equipped with a tune-o-matic or adjusto-matic bridge, but you want to use a jazz/jag or mustang bridge. here's one way of doing it that doesn't require any mods to the body:
1) remove the legs from your jazz/jag/stang bridge by bending them with a pair of pliers. it must be a reissue bridge. a vintage bridge has the legs welded in place and are therefore much harder to remove, plus it's just stupid to ruin a valuable vintage part. you could still use vintage saddles though.
2) measure the posts of your tune-o-matic bridge and find an appropriately sized drill bit (you might want to go slightly bigger to give the bridge base a little room to wiggle, but try an exact fit first)
3) drill out the holes in the jm/jag/stang bridge base
4) drop the bridge in place. done!
if your guitar has a 12" radius fretboard, you'll want to use jazz/jag saddles or the adjustable modified mustang saddles, since mustang saddles are made for 7.25" radius.
or if you're anything like me, you could do it the hard way, and file down the underside of the mustang saddles to adjust the radius permanently.
important! this applies to tune-o-matic/adjusto-matic bridges with the larger bridge posts (see pics below). if you have a nashville or ABR type bridge, the holes left in the bridge base when you remove the legs will be too large for the posts. it may work on those bridges too, but i haven't tried it. i suspect that the bridge will have too much wiggle room and you may run into tuning and/or buzzing issues.
the result of this mod is a bridge that has the stability and sustain of a tune-o-matic, and the smoother trem action of a jazz/jag/mustang bridge. and it will sound like a 50/50 mix of the two.
here's what it looks like:
